VXLAN support

This patch is implementing support for VXLAN tunnel
type for OVS plugin.

Refactored neutron::plugins::ml2::validate_* to functions
so they can be used in other manifests in module.

Change-Id: Ic51ed794b27069809a151dc365ca108c8300c277
(cherry picked from commit 766561df83ea414ccf82423414863f30c6ce0761)