Add abc support to generate_methods

This patch modifies generate_methods to add methods even if
they're defined in the parent classes as the abstract methods.

Change-Id: Iad5655084b32519e18107345a56cdb35f2f96bc8
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Add API handlers for chains and ruels to MiodnetPluginV2

This patch adds API handler mixin for chains and rules to
MidonetPluinV2 with the unit test of the POST requests.

Change-Id: Id7f87a815aa0c1eaed60656e60e01b4215dfd18f
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>