Add the introduce of the command "magnum help ca-show"

Currently, when executing "magnum help ca-show", there is only the use
of the commmand and isn't the introduce about the command.

This patch add the introduce of "magnum help ca-show"

Change-Id: I70bcf1ce015c4ee825062ff5bd15f2a21795c0a4
Closes-Bug: #1507874

magnumclient/v1/shell.py

@@ -277,6 +277,7 @@ def do_baymodel_list(cs, args):
            metavar='<bay>',
            help='ID or name of the bay.')
 def do_ca_show(cs, args):
+    """Show details about the CA certificate for a bay."""
     bay = cs.bays.get(args.bay)
     if bay.status not in ['CREATE_COMPLETE', 'UPDATE_COMPLETE']:
         print('Bay status for %s is: %s. We can not create a %s there'