ovn-migration-mtu: Support providing project/user domain by name
ovn-migration-mtu: Support providing project/user domain by name

The domain name environment variables `OS_USER_DOMAIN_NAME` and
`OS_PROJECT_DOMAIN_NAME` will be consumed when present.

Otherwise the current behaviour of using *_DOMAIN_ID environment
variables with fall back to the domain with literal ID of
`default` is retained.

Change-Id: I926a160914c16792d7911810cfded41d29fcf282
Closes-Bug: #1887778

mahak/neutron,mahak/neutron,mahak/neutron,openstack/neutron,openstack/neutron,openstack/neutron