Support networking-generic-switch in the overcloud

The networking-generic-switch project [1] provides a modular Neutron mechanism driver, covering a wide range of switches via the Netmiko library. It is commonly used in combination with ironic to provide advanced features, relying on switch management, such as multi-tenancy. Initially created for testing and CI, it is now used in several production deployments.

The python-networking-generic-switch package exists in RDO as of Queens.

[1] https://github.com/openstack/networking-generic-switch