(SIMP-10204) Ensure testing with CentOS 8.4 (#44)

- update the EL8 nodes in the acceptance tests
  to run with generic/centos8 to make sure they using
  CentOS 8.4
- update oel7 box to generic from onyxpoint in nodeset

SIMP-10204 #comment issue updates
SIMP-10226

spec/acceptance/nodesets/default.yml

@@ -7,15 +7,15 @@
 -%>
 HOSTS:
   el7:
-    roles: # migrated from now-removed el6 node(s)
+    roles:
     - default
     platform: el-7-x86_64
     box: centos/7
     hypervisor: <%= hypervisor %>
 
   el8:
     platform: el-8-x86_64
-    box: centos/8
+    box: generic/centos8
     hypervisor: <%= hypervisor %>
 
 CONFIG:

spec/acceptance/nodesets/oel.yml

@@ -10,7 +10,7 @@ HOSTS:
     roles: # migrated from now-removed el6 node(s)
     - default
     platform:   el-7-x86_64
-    box:        onyxpoint/oel-7-x86_64
+    box:        generic/oracle7
     hypervisor: <%= hypervisor %>
 
   oel8: