Adds support for a VTEP tunnel zone type

This patch adds support for a VTEP tunnel zone type,
for network topologies that need to configure a physical
VTEP.

Fixes: MN-2573

Signed-off-by: Alex Bikfalvi <alex.bikfalvi@midokura.com>
Merge pull request #233 from alexbikfalvi/master

Adds support for a VTEP tunnel zone type