--property should be required in os unset commands

The three commands below
  openstack volume unset
  openstack snapshot unset
  openstack volume type unset

Should have --property as a required argument,
not optional. The reason is the command will not work
without --property.

Closes-Bug: #1475872
Change-Id: Ib9e29392472db38982cc2817af2dd5055f5a01ca

openstackclient/volume/v1/snapshot.py

@@ -263,6 +263,7 @@ class UnsetSnapshot(command.Command):
             default=[],
             help='Property to remove from snapshot '
                  '(repeat to remove multiple values)',
+            required=True,
         )
         return parser
 

openstackclient/volume/v1/type.py

@@ -166,6 +166,7 @@ class UnsetVolumeType(command.Command):
             default=[],
             help='Property to remove from volume type '
                  '(repeat option to remove multiple properties)',
+            required=True,
         )
         return parser
 

openstackclient/volume/v1/volume.py

@@ -437,6 +437,7 @@ class UnsetVolume(command.Command):
             default=[],
             help='Property to remove from volume '
                  '(repeat option to remove multiple properties)',
+            required=True,
         )
         return parser