image cache clear: fix value of default target

When using the "openstack image cache clear" command, the "clear_cache"
method from the OpenStack SDK is used. It expects its only argument to
be one of "both", "cache" or "queue". However, when passing neither
"--cache" nor "--queue", it is currently passed None as a value. Fix
this by specifying "both" as the default value to be passed.

Change-Id: I17c6e3d435a84b4ba453845086ff3fe272b54f58

Author: CyrilRoelandteNovance

openstackclient/image/v2/cache.py

@@ -194,13 +194,15 @@ def get_parser(self, prog_name):
             "--cache",
             action="store_const",
             const="cache",
+            default="both",
             dest="target",
             help=_("Clears all the cached images"),
         )
         parser.add_argument(
             "--queue",
             action="store_const",
             const="queue",
+            default="both",
             dest="target",
             help=_("Clears all the queued images"),
         )

openstackclient/tests/unit/image/v2/test_cache.py

@@ -184,13 +184,13 @@ def setUp(self):
     def test_cache_clear_no_option(self):
         arglist = []
 
-        verifylist = [('target', None)]
+        verifylist = [('target', 'both')]
 
         parsed_args = self.check_parser(self.cmd, arglist, verifylist)
         self.cmd.take_action(parsed_args)
 
         self.assertIsNone(
-            self.image_client.clear_cache.assert_called_with(None)
+            self.image_client.clear_cache.assert_called_with('both')
         )
 
     def test_cache_clear_queue_option(self):