Add Docker integration for OVN.

Docker multi-host networking is now part of
Docker 1.9.

This commit adds two drivers for OVN integration
with Docker. The first driver is a pure overlay driver
that does not need OpenStack integration. The second driver
needs OVN+OpenStack.

The description of the Docker API exists here:
https://github.com/docker/libnetwork/blob/master/docs/remote.md

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@ovn.org>