clarify usage of upgrade_levels group

clarify usage of upgrade_levels group especially to note
other than upgrade_levels.compute that can't use 'auto'.

Closes-Bug: 1781916

Change-Id: I23ae3895d63a4f6aa40b4a17822270da7cd5b9e3

nova/conf/upgrade_levels.py

@@ -61,23 +61,113 @@ Possible values:
   'liberty'.
 """),
     cfg.StrOpt('cells',
-        help='Cells RPC API version cap'),
+        help="""
+Cells RPC API version cap.
+
+Possible values:
+
+* By default send the latest version the client knows about
+* A string representing a version number in the format 'N.N';
+  for example, possible values might be '1.12' or '2.0'.
+* An OpenStack release name, in lower case, such as 'mitaka' or
+  'liberty'.
+"""),
     cfg.StrOpt('intercell',
-        help='Intercell RPC API version cap'),
+        help="""
+Intercell RPC API version cap.
+
+Possible values:
+
+* By default send the latest version the client knows about
+* A string representing a version number in the format 'N.N';
+  for example, possible values might be '1.12' or '2.0'.
+* An OpenStack release name, in lower case, such as 'mitaka' or
+  'liberty'.
+"""),
     cfg.StrOpt("cert",
-        help='Cert RPC API version cap'),
+        help="""
+Cert RPC API version cap.
+
+Possible values:
+
+* By default send the latest version the client knows about
+* A string representing a version number in the format 'N.N';
+  for example, possible values might be '1.12' or '2.0'.
+* An OpenStack release name, in lower case, such as 'mitaka' or
+  'liberty'.
+"""),
     cfg.StrOpt("scheduler",
-        help='Scheduler RPC API version cap'),
+        help="""
+Scheduler RPC API version cap.
+
+Possible values:
+
+* By default send the latest version the client knows about
+* A string representing a version number in the format 'N.N';
+  for example, possible values might be '1.12' or '2.0'.
+* An OpenStack release name, in lower case, such as 'mitaka' or
+  'liberty'.
+"""),
     cfg.StrOpt('conductor',
-        help='Conductor RPC API version cap'),
+        help="""
+Conductor RPC API version cap.
+
+Possible values:
+
+* By default send the latest version the client knows about
+* A string representing a version number in the format 'N.N';
+  for example, possible values might be '1.12' or '2.0'.
+* An OpenStack release name, in lower case, such as 'mitaka' or
+  'liberty'.
+"""),
     cfg.StrOpt('console',
-        help='Console RPC API version cap'),
+        help="""
+Console RPC API version cap.
+
+Possible values:
+
+* By default send the latest version the client knows about
+* A string representing a version number in the format 'N.N';
+  for example, possible values might be '1.12' or '2.0'.
+* An OpenStack release name, in lower case, such as 'mitaka' or
+  'liberty'.
+"""),
     cfg.StrOpt('consoleauth',
-        help='Consoleauth RPC API version cap'),
+        help="""
+Consoleauth RPC API version cap.
+
+Possible values:
+
+* By default send the latest version the client knows about
+* A string representing a version number in the format 'N.N';
+  for example, possible values might be '1.12' or '2.0'.
+* An OpenStack release name, in lower case, such as 'mitaka' or
+  'liberty'.
+"""),
     cfg.StrOpt('network',
-        help='Network RPC API version cap'),
+        help="""
+Network RPC API version cap.
+
+Possible values:
+
+* By default send the latest version the client knows about
+* A string representing a version number in the format 'N.N';
+  for example, possible values might be '1.12' or '2.0'.
+* An OpenStack release name, in lower case, such as 'mitaka' or
+  'liberty'.
+"""),
     cfg.StrOpt('baseapi',
-        help='Base API RPC API version cap')
+        help="""
+Base API RPC API version cap.
+
+Possible values:
+
+* By default send the latest version the client knows about
+* A string representing a version number in the format 'N.N';
+  for example, possible values might be '1.12' or '2.0'.
+* An OpenStack release name, in lower case, such as 'mitaka' or
+  'liberty'.
+""")
 ]