Change default network backend driver to ML2/OVN

This patch is changing the default network backend driver in DevStack to
OVN.

This is a long effort from the Neutron community that has been
discussed on previous PTGs and agreed upon.

A similar patch to this one [0] have been merged in the past but got
reverted [1] because it did break some zuul jobs. This one also include
fixes for such jobs and was verified at [2]

[0] https://review.opendev.org/c/openstack/devstack/+/735097
[1] https://review.opendev.org/c/openstack/neutron/+/775632
[2] https://review.opendev.org/c/zuul/zuul-jobs/+/791117

Change-Id: I8c2be82f33ed9f6c36f5509b3b210ee1a38e87ca
Signed-off-by: Lucas Alvares Gomes <lucasagomes@gmail.com>
4 files changed