deprecate external_network_bridge and gateway_external_network_id options

[zhongshengping]

See following neutron-l3-agent.log. The external_network_bridge option is deprecated and will be removed in Ocata.

2017-05-22 15:44:02.737 4832 DEBUG neutron_lib.callbacks.manager [req-07f573f5-4315-4700-b064-516ae234d40f - - - - -] Subscribe: <function remove_router at 0x7f7389355b90> router after_delete subscribe /openstack/venvs/neutron-testing/local/lib/python2.7/site-packages/neutron_lib/callbacks/manager.py:41
2017-05-22 15:44:02.738 4832 WARNING oslo_config.cfg [req-07f573f5-4315-4700-b064-516ae234d40f - - - - -] Option "external_network_bridge" from group "DEFAULT" is deprecated for removal. Its value may be silently ignored in the future.
2017-05-22 15:44:02.738 4832 WARNING oslo_config.cfg [req-07f573f5-4315-4700-b064-516ae234d40f - - - - -] Option "gateway_external_network_id" from group "DEFAULT" is deprecated for removal. Its value may be silently ignored in the future.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/466998

[OpenStack Infra (hudson-openstack)]

Change abandoned by zhongshengping ([e-mail]) on branch: master
Review: https://review.openstack.org/466998