Revert "Change Neutron's default ML2 driver to OVN"

This reverts commit 5c304d817682d6c807b532b50a2f105479ac3fa2.

Reason for revert: There are more things to fix/move like done in 791085 and 791282 Also let's change all required default in devstack scripts instead of devstack's zuul job side. Basically do this change without any change in .zuul.yaml

Change-Id: Ie0f59d1b9a4b97ad9fd8131819054dfb616f31fd
2 files changed