Consume openstackclient.common.command in subnet/port

Follow-up patch of https://review.openstack.org/#/c/269613/
network/v2/subnet and port still use cliff classes directly.
This patch fixes it.

Change-Id: If9d90e5151ece7f4cf1e0d6fd2f32919865f2f2e

Author: Akihiro Motoki

openstackclient/network/v2/port.py

@@ -13,7 +13,7 @@
 
 """Port action implementations"""
 
-from cliff import command
+from openstackclient.common import command
 
 
 class DeletePort(command.Command):

openstackclient/network/v2/subnet.py

@@ -13,8 +13,7 @@
 
 """Subnet action implementations"""
 
-from cliff import lister
-
+from openstackclient.common import command
 from openstackclient.common import utils
 
 
@@ -31,7 +30,7 @@ def _format_allocation_pools(data):
 }
 
 
-class ListSubnet(lister.Lister):
+class ListSubnet(command.Lister):
     """List subnets"""
 
     def get_parser(self, prog_name):