Fix auth version detection

If OS_CLOUD is not set the defalt value is parsed which is empty string. This
is wrong, as it deactivates all other environment variables for cloud
configuration.

Change-Id: I48452bb5300fcc46047f8e94501bba1c42629ea1
Story: 2001158
Task: 4890
(cherry picked from commit fecf0e3d66)

monascaclient/shell.py

@@ -55,6 +55,7 @@ class MonascaShell(shell.OpenStackShell):
             description,
             version
         )
+        parser.set_defaults(cloud=None)
         parser = auth.build_auth_plugins_option_parser(parser)
         parser = self._append_monasca_args(parser)
         return parser