schema: Completely drop our JSON Schema 'id' properties

[wking]

We're using JSON Schema draft-04, as declared by our $schema properties. In draft-04, the id keyword alters the resolution scope. But our current $ref values use JSON Pointers with relative references like defs-linux.json#/definitions/Device that ignore the ids.

By draft-07, id has become $id, and the docs say:

The root schema of a JSON Schema document SHOULD contain an "$id" keyword with a URI (containing a scheme).

But since xeipuuv/gojsonschema@83a7f6369d5, including any URI that cannot be retrieved generates an error:

$ ./validate config-schema.json test/config/good/minimal.json
Could not read schema from HTTP, response status is 404 Not Found

While a root id entry would be nice, we don't currently host these anywhere with a useful URI. We could use GitHub links like this, but then testing pull requests would be difficult.

By draft-07, the purpose of internal $id entries is clearly explained:

Providing a plain name fragment enables a subschema to be relocated within a schema without requiring that JSON Pointer references are updated.

We don't need that, because we control all the references. In the infrequent event of a subschema move, we can update the consuming references in the same commit.

The draft-07 $ref docs also explain that $ref targets may be URNs:

The URI is not a network locator, only an identifier. A schema need not be downloadable from the address if it is a network-addressable URL, and implementations SHOULD NOT assume they should perform a network operation when they encounter a network-addressable URI.

I haven't found analogous wording for $id, but it's possible that gojsonschema is being overly agressive with its attempted retrievals.

This commit removes all of our id entries. The resulting JSON Schema is valid (regardless of where you host it) and does not generate the 404s.

If/when we merge/release this it should address opencontainers/runc#1680.

[crosbymichael]

LGTM

[tianon]

LGTM