Refactor init_l3 to separate router port use case

Refactor init_l3 to separate router port use case

Future work will extend init_l3 with more code specific to router
ports.  It makes sense to separate these out in to one basic method
with basic L3 and another for router port specific logic.

Change-Id: Iec9a46cd0490c4f48bb306083711ff0c5e70ba87
Partially-Implements: blueprint address-scopes