Fix KeyError except on router_info in FW Agent

Fix KeyError except on router_info in FW Agent

The workflow of creating a firewall when a router without any i/f is present in that tenant
causes a KeyError on the FWAgent. The issue occurs as such routers are present in the list of
routers returned by get_routers() but are not populated in the router_info dict. Adding a check
before accessing the dict to prevent the exception. When an i/f is added to such routers -
_router_added processing in the l3agent will populate the router_info dict and the FWAgent
also picks this up in that context.

Change-Id: I5ea22e88a46e62372a0462e9cb958a08dd9f4c7d
Closes-Bug: #1289643