Let router update to specify external gw net ID
Let router update to specify external gw net ID

One can also update the external gateway network ID on a router.
Let's allow that.

Change-Id: If1026c897687d285f01bcfd847bfc004fa720887

jsmartin/shade,dtroyer/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk,openstack/python-openstacksdk,jsmartin/shade,stackforge/python-openstacksdk,openstack-infra/shade,dtroyer/python-openstacksdk,openstack-infra/shade