bug #1042028
Allows user to invoke L3-agent in a way that limits the agent to
implementing only routers associated with a particular external network.
Thus, it is possible to have a deployment with multiple external networks,
since you can run one agent per external network.
Also makes l3-agent pay attention to router admin_state_up field.
Change-Id: Ia79d29301530718bc381c8e5a9d197de9452bce2