Add API handlers for bridges to MiodnetPluginV2

This patch adds API handler mixin for bridges to MidonetPluinV2
with the unit test of the POST request.

Change-Id: Ieb2da31ff91f09ef43887d4110f96448954a452c
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>