Change Neutron's default ML2 driver to OVN

As part of the Victoria PTG the Neutron community [0] agreed on changing
the default backend driver from ML2/OVS to ML2/OVN in DevStack. A lot of
changes have been submitted towards this goal including but not limted
to:

* Moving the OVN module to DevStack:
  https://review.opendev.org/c/openstack/devstack/+/734621

* Updating the OVN module to use distro packages instead of compiling
  OVN from source: https://review.opendev.org/c/openstack/devstack/+/763402o

And now this patch is changing the the actual Q_AGENT,
Q_ML2_TENANT_NETWORK_TYPE and Q_ML2_PLUGIN_MECHANISM_DRIVERS values in
devstack to what is expected by OVN as well as updating the Zuul
templates to enable the OVN services.

[0] https://etherpad.opendev.org/p/neutron-victoria-ptg

Change-Id: I92054ce9d2ab7a42746ed5dececef583b0f8a833
Signed-off-by: Lucas Alvares Gomes <lucasagomes@gmail.com>
2 files changed