gbp: add anonymous l3-out subnets

gbp: add anonymous l3-out subnets

An anonymous l3-out subnet is a locally attached l3-out subnet, and
differs from regular l3-out subnets in the way adjacencies are managed.
It is required for the anonymous l3-out external interfaces to correctly
classify locally attached l3-out hosts.

Type: feature

Change-Id: Ie7bc88b1f22abc4d0b46db5f3cfbf208bc53ba5f
Signed-off-by: Benoît Ganne <bganne@cisco.com>