policy: replace enums with oneOfs in the schemas

The reason is that this way, we can add a description for each of the
options.

gateway/src/apicast/policy/caching/apicast-policy.json

@@ -1,32 +1,47 @@
 {
   "$schema": "http://apicast.io/policy-v1/schema#manifest#",
   "name": "Caching policy",
-  "description":
-    ["Configures a cache for the authentication calls against the 3scale ",
-     "backend. The 3scale backend can authorize (status code = 200) and deny ",
-     "(status code = 4xx) calls. When it fails, it returns a 5xx code. ",
-     "This policy support three kinds of caching: \n",
-     " - Strict: it only caches authorized calls. Denied and failed calls ",
-     "invalidate the cache entry.\n",
-     " - Resilient: caches authorized and denied calls. Failed calls do not ",
-     "invalidate the cache. This allows us to authorize and deny calls ",
-     "according to the result of the last request made even when backend is ",
-     "down.\n",
-     "- Allow: caches authorized and denied calls. When backend is ",
-     "unavailable, it will cache an authorization. In practice, this means ",
-     "that when backend is down _any_ request will be authorized unless last ",
-     "call to backend for that request returned 'deny' (status code = 4xx). ",
-     "Make sure to understand the implications of that before using this ",
-     "mode. It makes sense only in very specific use cases.\n",
-     "- None: disables caching."],
+  "description": "Configures a cache for the authentication calls against the 3scale backend",
   "version": "0.1",
   "configuration": {
     "type": "object",
     "properties": {
       "caching_type": {
         "description": "Caching mode",
-        "type": "string",
-        "enum": ["resilient", "strict", "allow", "none"]
+        "oneOf": [
+          {
+            "const": "resilient",
+            "description": [
+              "Caches authorized and denied calls. Failed calls do not ",
+              "invalidate the cache. This allows us to authorize and deny ",
+              "calls according to the result of the last request made even ",
+              "when backend is down."
+            ]
+          },
+          {
+            "const": "strict",
+            "description": [
+              "It only caches authorized calls. Denied and failed calls ",
+              "invalidate the cache entry."
+            ]
+          },
+          {
+            "const": "allow",
+            "description": [
+              "Caches authorized and denied calls. When backend is ",
+              "unavailable, it will cache an authorization. In practice, this ",
+              "means that when backend is down _any_ request will be ",
+              "authorized unless last call to backend for that request ",
+              "returned 'deny' (status code = 4xx).\n",
+              "Make sure to understand the implications of that before using this ",
+              "mode. It makes sense only in very specific use cases."
+            ]
+          },
+          {
+            "const": "none",
+            "description": "Disables caching."
+          }
+        ]
       }
     }
   }

gateway/src/apicast/policy/echo/apicast-policy.json

@@ -13,9 +13,17 @@
         "type": "integer"
       },
       "exit": {
-        "description": "Exit mode. 'request' interrupts the processing of the request, 'phase' only skips the rewrite phase.",
-        "type": "string",
-        "enum": ["request", "phase"]
+        "description": "Exit mode",
+        "oneOf": [
+          {
+            "const": "request",
+            "description": "Interrupts the processing of the request."
+          },
+          {
+            "const": "set",
+            "description": "Only skips the rewrite phase."
+          }
+        ]
       }
     }
   }

gateway/src/apicast/policy/headers/apicast-policy.json

@@ -17,9 +17,35 @@
           "type": "object",
           "properties": {
             "op": {
-              "description": "Operation to be applied",
-              "type": "string",
-              "enum": ["add", "set", "push"]
+              "oneOf": [
+                {
+                  "const": "add",
+                  "description": [
+                    "When the header is not set, it does nothing.\n",
+                    "When the header is set, it creates a new header with the ",
+                    "same name and the given value."
+                  ]
+                },
+                {
+                  "const": "set",
+                  "description": [
+                    "When the header is not set, creates it with the given ",
+                    "value.\n",
+                    "When the header is set, replaces its value with the ",
+                    "given one.\n",
+                    "Deletes a header when the value is ''."
+                  ]
+                },
+                {
+                  "const": "push",
+                  "description": [
+                    "When the header is not set, creates it with the given ",
+                    "value.\n",
+                    "When the header is set, it creates a new header with the ",
+                    "same name and the given value."
+                  ]
+                }
+              ]
             },
             "header": {
               "description": "Header to be modified",

gateway/src/apicast/policy/url_rewriting/apicast-policy.json

@@ -19,8 +19,16 @@
           "properties": {
             "op": {
               "description": "Operation to be applied (sub or gsub)",
-              "type": "string",
-              "enum": ["sub", "gsub"]
+              "oneOf": [
+                {
+                  "const": "sub",
+                  "description": "Substitutes the first match of the regex applied."
+                },
+                {
+                  "const": "gsub",
+                  "description": "Substitutes all the matches of the regex applied."
+                }
+              ]
             },
             "regex": {
               "description": "Regular expression to be matched",