OVN to UCA codename code does not cope with the focal-ovn-22.03 overlay pocket

[fnordahl]

In the light of OVN 22.03 being made available on Focal through a separate UCA overlay pocket (ref juju/charm-helpers#738) we may have a situation where the deployed OpenStack under test is focal-ussuri while OVN is 22.03, and not 20.03 as was released with Focal.

The current versioning code would not be able to cope with that and I wonder if we need to decouple it from UCA pocket codenames:

zaza-openstack-tests/zaza/openstack/utilities/os_versions.py

Lines 126 to 135 in 778feeb

	OVN_CODENAMES = OrderedDict([
	('train',
	['2.12']),
	('ussuri',
	['20.03']),
	('victoria',
	['20.06']),
	('wallaby',
	['20.12']),
	])

The OVN test code itself does currently not rely on version fences, but I think the automated openstack upgrade code might.