Big Switch: Separate L3 functions into L3 service

Separate the L3 functions from the core Big Switch
plugin into an L3 service plugin. This is will allow
the L3 functions to be used in ML2 deployments that
use the Big Switch ML2 driver.

DocImpact

Implements: blueprint bsn-l3-service-plugin
Change-Id: I889db0047fb9a85f02d1fb95b9c099e9243a5bb5
Merge "Big Switch: Separate L3 functions into L3 service"