VLAN support for Distributed Virtual Router (DVR)

This blueprint proposes enhancement to existing
DVR infrastructure,  enabling it to embrace
VLAN networks as dvr interfaces.  It also proposes
enhancements that will enable DVR to route between
VLAN and VXLAN/GRE networks, thereby closing a
parity gap of DVR with centralized router.

DocImpact
Blueprint: neutron-ovs-dvr-vlan
Change-Id: I0273af2f6ea0084aa0943ebcee5875b3c9a472ba