Make cluster config --force a boolean

Option --force of openstack coe cluster config is a boolean and should not
take an argument. Fix message in argument description.

Change-Id: Ic526589a8f5c11b13e16b080a04e262d8393c387
Closes-Bug: #1721267
(cherry picked from commit 92fa44e1d9)

magnumclient/osc/v1/clusters.py

@@ -263,9 +263,10 @@ class ConfigCluster(command.Command):
             help=_('Directory to save the certificate and config files.'))
         parser.add_argument(
             '--force',
-            metavar='<force>',
+            action='store_true',
+            dest='force',
             default=False,
-            help=_('Directory to save the certificate and config files.'))
+            help=_('Overwrite files if existing.'))
 
         return parser