Add support for flat provider network

[msimonin]

Neutron is currently deployed with the default parameters provided by Kolla. As a consequence tenant networks are enabled by default. In the perspective of running Openstack over Openstack I'm tempting to switch to a simpler model for neutron deployment and use a flat provider network. IPs would be taken on the kavlan ips pool. No tenant networks / floating ip will be supported anymore. This way we avoid the cost of the overlay network for the under cloud (by extension we also could avoid it for the over cloud if needed). This configuration should be eased by the fact that kolla/newton allows custom config to be placed along with the deployment files.

[msimonin]

Not the priority.