OVS-agent: Introduce Ryu based OpenFlow implementation

OVS-agent: Introduce Ryu based OpenFlow implementation

Introduce an alternative OpenFlow implementation, "native",
implemented using Ryu ofproto python library from Ryu SDN Framework.
Make it selectable with of_driver=native agent option.
The aim is to replace the existing ovs-ofctl based implementation
eventually.

It introduces node-local OpenFlow controller embedded in
OVS agent.  Benefits include:
* Reduce the overhead of invoking ovs-ofctl command (and associated
  rootwrap)
* Make future uses of OpenFlow asynchronous messages (e.g. Packet-In,
  Port-Status, etc) easier
* Make XenAPI integration simpler

Highlights:
* Switch to OpenFlow 1.3.
* Make OVS-agent act as an OpenFlow controller
* Configure OVS on the node to connect to the controller

DocImpact

Implements: blueprint ovs-ofctl-to-python
Co-Authored-by: IWAMOTO Toshihiro <iwamoto@valinux.co.jp>
Change-Id: I02e65ea7c6083b2c0a686fed2ab04da4d92b21a3