(SIMP-10204) Ensure testing with CentOS 8.4 (#137)

- update the EL8 nodes in the acceptance tests
  to run with generic/centos8 to make sure they using
  CentOS 8.4
- update oel to use generic instead of onyxpoint box

SIMP-10204 #comment update compliance_markup
SIMP-10212 #close

spec/acceptance/nodesets/centos.yml

@@ -17,7 +17,7 @@ HOSTS:
     roles:
       - client
     platform:   el-8-x86_64
-    box:        centos/8
+    box:        generic/centos8
     hypervisor: <%= hypervisor %>
 CONFIG:
   log_level: verbose

spec/acceptance/nodesets/oel.yml

@@ -11,7 +11,7 @@ HOSTS:
       - server
       - default
     platform:   el-7-x86_64
-    box:        onyxpoint/oel-7-x86_64
+    box:        generic/oracle7
     hypervisor: <%= hypervisor %>
 
   oel8: