Extraroute extension support for nuage plugin

Nuage's VSP supports adding static-route to L3 Domain
which fits nicely with extraroute extension supported
by openstack's neutron. This set of change enables that.

Change-Id: Icd4d95b9077056c33c7509fe45966ff2a04cd923
Implements: blueprint extraroute-ext-support-for-nuage-plugin
Merge "Extraroute extension support for nuage plugin"