Replace VTEP's tunnel-zone-id property with tunnel-zone ObjectRef.

This allows users to specify a tunnel zone by alias rather than by
typing out the tunnel zone's UUID.

Fixes: MN-2422
Signed-off-by: Brandon Berg <bberg@midokura.jp>
Merge pull request #196 from brandon-berg/vtep-tunnel-zone

Replace VTEP's tunnel-zone-id property with tunnel-zone ObjectRef.