Fix errors for "volume type unset" command

Normally, we can unset multi properties but the "volume type unset"
command could not, because the action "append" was missed. So I add
it and also fix the unit test and doc in this patch.

Change-Id: I20470f2b7bb2a8d7f292cea498826669c3418c77

doc/source/command-objects/volume-type.rst

@@ -150,7 +150,7 @@ Unset volume type properties
 .. code:: bash
 
     os volume type unset
-        [--property <key>]
+        [--property <key> [...] ]
         [--project <project>]
         [--project-domain <project-domain>]
         <volume-type>

openstackclient/tests/volume/v2/test_type.py

@@ -397,17 +397,19 @@ class TestTypeUnset(TestType):
     def test_type_unset(self):
         arglist = [
             '--property', 'property',
+            '--property', 'multi_property',
             self.volume_type.id,
         ]
         verifylist = [
-            ('property', 'property'),
+            ('property', ['property', 'multi_property']),
             ('volume_type', self.volume_type.id),
         ]
 
         parsed_args = self.check_parser(self.cmd, arglist, verifylist)
 
         result = self.cmd.take_action(parsed_args)
-        self.volume_type.unset_keys.assert_called_once_with('property')
+        self.volume_type.unset_keys.assert_called_once_with(
+            ['property', 'multi_property'])
         self.assertIsNone(result)
 
     def test_type_unset_project_access(self):

openstackclient/volume/v2/volume_type.py

@@ -263,6 +263,7 @@ class UnsetVolumeType(command.Command):
         parser.add_argument(
             '--property',
             metavar='<key>',
+            action='append',
             help=_('Remove a property from this volume type '
                    '(repeat option to remove multiple properties)'),
         )