Make CONFIG_NEUTRON_OVS_VXLAN_UDP_PORT work with ML2

Group NEUTRON_OVS_PLUGIN_AND_AGENT_VXLAN does not get loaded when
ML2 with VXLAN is used. This patch is fixing it.

Change-Id: I1528726bb8380c8d9ed20855a62e8198086b1a38
Merge "Make CONFIG_NEUTRON_OVS_VXLAN_UDP_PORT work with ML2"