Merge rbac_auth with rbac_rule_validation

Currently, rbac_auth doesn't do much: It decentralizes logic
that can be easily merged into rbac_rule_validation without
doing anything authentication-related. All rbac_auth does is:

  1) Construct RbacPolicyParser and check whether a given role
     is allowed to perform a given policy action.
  2) Dump some info to LOG
  3) Catch some exceptions

Thus, there's no justification for keeping rbac_auth. It doesn't
provide a high-enough level of abstraction to warrant being used.
It should be removed and its logic inserted in rbac_rule_validation.

Change-Id: I756175ea28ec11f24150f46d5ae4c2f64499a0ea
Closes-Bug: #1681459
diff --git a/releasenotes/notes/merge-rbac-auth-with-rbac-rule-validation-5d7c286788a95ee9.yaml b/releasenotes/notes/merge-rbac-auth-with-rbac-rule-validation-5d7c286788a95ee9.yaml
new file mode 100644
index 0000000..b96c73a
--- /dev/null
+++ b/releasenotes/notes/merge-rbac-auth-with-rbac-rule-validation-5d7c286788a95ee9.yaml
@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    Merges `rbac_auth` with `rbac_rule_validation`, because `rbac_auth`
+    decentralized logic from `rbac_rule_validation` without providing any
+    authentication-related utility. This change facilitates code maintenance
+    and code readability.