Switch the default Neutron ML2 driver to OVN
This patch is changing the default driver used in Packstack from
ML2/OVS to ML2/OVN. It also flips jobs running with OVS to OVN and
viceversa while keeping the lbaas scenario with OVS.
Also, the encapsulation used by default changes from vxlan to
Signed-off-by: Daniel Alvarez <firstname.lastname@example.org>