Fix the attribute map of tunnel zone

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Fix Tunnelzone class

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Refactor the tunnel zone handler to to be separated

This patch separates the tunnel zone handler out from MidonetPluginV2
class creating a mixin for it.

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