L3 Agent can handle many external networks

With this patch a l3 agent can handle
any networks by setting the neutron
parameter external_network_bridge
and gateway_external_network_id to
empty.

cherry picked from commit b5092af696131e611e20f17c6b9aa4f321a854df

Related-Bug: #1234750
Related-Bug: #1194350
Change-Id: Id260a239df23951da730513f40cda490002effc3
Merge "L3 Agent can handle many external networks" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable