Deprecate cert option

Mark cert_topic option for deprecation in future [1] & [2].

[1] Removal of nova-cert service as indicated by commit:789edad0e811d866551bec18dc7729541105f59d
    https://review.openstack.org/#q,789edad0e811d866551bec18dc7729541105f59d,n,z

[2] https://wiki.openstack.org/wiki/ConfigOptionsConsistency

Implements: Blueprint "centralize-config-options-newton"

Change-Id: Id7a1fc943cbe6d860a50d3cc776717b55351004b

nova/conf/cert.py

@@ -1,5 +1,3 @@
-# needs:check_deprecation_status
-
 # Copyright 2016 IBM Corp.
 # Copyright 2016 OpenStack Foundation
 # All Rights Reserved.
@@ -20,6 +18,12 @@ from oslo_config import cfg
 
 cert_opts = [
     cfg.StrOpt("cert_topic",
+        deprecated_for_removal=True,
+        deprecated_reason="""
+Since the nova-cert service is marked for deprecation, the feature to change
+RPC topic that cert nodes listen may be removed as early as the 15.0.0
+Ocata release.
+""",
         default="cert",
         help="""
 Determines the RPC topic that the cert nodes listen on. For most deployments

releasenotes/notes/deprecate_os_cert-f0aa07bab1a229aa.yaml

@@ -2,7 +2,9 @@
 deprecations:
 
   - The ``/os-certificates`` API is deprecated, as well as the
-    ``nova-cert`` service which powers it. This is a vestigial part of
+    ``nova-cert`` service which powers it. The related config option
+    ``cert_topic`` is also now marked for deprecation and may be removed as
+    early as 15.0.0 Ocata release. This is a vestigial part of
     the Nova API that existed only for EC2 support, which is now
     maintained out of tree. It does not interact with any of the rest
     of nova, and should not just be used as a certificates as a