Implement MidoNet plugin for Icehouse (Part 1)

Update the plugin to reflect the newest MidoNet API that mostly
mirrors the Neutron API.  Most of the Neutron API can now be
simply passed through to MidoNet, making the plugin easier to
maintain, and it provides better data synchronization since
most of MidoNet API operations are atomic.

Change-Id: Ie4a57a15ec6ea58a594399b7628ea6a3624f694a
Implements: blueprint midonet-plugin
Signed-off-by: Ryu Ishimoto <ryu@midokura.com>