Since 20.09, OVN supports VXLAN type for inter-chassis communication.
This patch also gets rid of no longer needed override for
NeutronNetworkType for OVN, moving the constraints into generic ML2
definition list. The constraints list is extended to include vxlan.
Depends-On: I81c016ba9c91282d1bebb40a282077e14ce4bd6b
Change-Id: I447458c344a8817f3cfacba06f3410d500ed1f59