Make explicit the network backend used in the CI jobs

All Neutron CI jobs (except for unit, functional and fullstack jobs),
have explicitly defined the network backend used:
- linuxbridge
- ovs
- ovn

That was discussed and approved during the Neutron CI meetings [1].

[1]https://meetings.opendev.org/meetings/neutron_ci/2021/neutron_ci.2021-06-15-15.00.log.html

Depends-On: https://review.opendev.org/c/openstack/neutron/+/797051
Change-Id: Ib14542311e9b1d49829bef54f433b8a04709a9fd
1 file changed