Add Networking-L2GW project

This repo will be used for the development of L2 Gateways initiatives
that integrate with Neutron.

In line with proposal [1], and as suggested in [2], this project will be
used to achieve loose coupling with Neutron, and to allow contributors to
experiment outside the core of the Neutron project.

[1] https://review.openstack.org/#/c/136835/
[2] https://wiki.openstack.org/wiki/Neutron-drivers

Change-Id: I5f4099a9613f82e7ba7e9478ea93dd5816c3cbc5