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.

Related-Bug: #1234750
Related-Bug: #1194350

Change-Id: Id260a239df23951da730513f40cda490002effc3
Merge "L3 Agent can handle many external networks"