Improve the help text for the cert options

The previous commit moved all the cert options to a central location.
This commit adds additional help text to those options.

Blueprint centralize-config-options

Change-Id: Ic4d15eebefced8478e36997294264eb1951fd464

nova/conf/cert.py

@@ -18,9 +18,42 @@ from oslo_config import cfg
 
 cert_topic_opt = cfg.StrOpt("cert_topic",
         default="cert",
-        help="The topic cert nodes listen on")
+        help="""
+Determines the RPC topic that the cert nodes listen on. The default is 'cert',
+and for most deployments there is no need to ever change it.
+
+Possible values:
+
+Any string.
+
+* Services which consume this:
+
+    ``nova-cert``
+
+* Related options:
+
+    None
+""")
+
 rpcapi_cap_opt = cfg.StrOpt("cert",
-        help="Set a version cap for messages sent to cert services")
+        help="""
+Specifies the maximum version for messages sent from cert services. This should
+be the minimum value that is supported by all of the deployed cert services.
+
+Possible values:
+
+Any valid OpenStack release name, in lower case, such as 'mitaka' or 'liberty'.
+Alternatively, it can be any string representing a version number in the format
+'N.N'; for example, possible values might be '1.12' or '2.0'.
+
+* Services which consume this:
+
+    ``nova-cert``
+
+* Related options:
+
+    None
+""")
 
 
 def register_opts(conf):