Function get_or_create_domain is required by designate-sink service,
but get lost during later changes, so add it back.
Otherwise, there will be "get_or_create_domain: command not found" error
during designate sink start.
Change-Id: I0433f83d02a2891c9bad4b9e26f93c4709ff1325
Closes-Bug: #1476575
5ed4be129c