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