Add OVS to OVN migration at the end of the regular gate test. This
adds only 5-10 minutes to each job and we want to confirm this
works from focal-ussuri and onwards as this is the point where we
recomend our end users to migrate from OVS to OVN.
Do ch-sync.
Merge after https://github.com/juju/charm-helpers/pull/511
Change-Id: I506f499c8c506d999e79f223f03a58abc29b8746
Depends-On: Ifa99988612eaaeb9d60a0d99db172f97e27cfc93