Add API handlers for ports to MidonetPluginV2

This patch adds API handlers for ports to MidonetPluginV2 with
a unit test of the port creation.

Change-Id: I4d9feb35d91adcc1bc3ea069e705ae53f6b5bf1a
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>