VXLAN support

This patch adds support for VXLAN for Neutron monolithic OVS plugin.

Change-Id: Ib5163b770817a9c1530fcbb7b2b10a8e214ab815
Fixes: rhbz#1021778
(cherry picked from commit d725f04300263b80284bf80b5a7ed15601905498)
Merge "VXLAN support" into havana