Add Midonet License extension

Fixes MN-2862

Change-Id: Ic76658430c1c3e098672cd38add65b7fddc87675
Merge "Add Midonet License extension"
Add an abstract class for tunnel zones

This patch adds the abstract class for tunnel zones and tunnel zone
hosts.

Change-Id: Iff5fffd071dbbbfea2c76150e96a10fad62ef486
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>