The interface code curretly strictly gates operation based on
information from `goal-state`. This information is however not
available when used with CMR.
Since the ovn-central charm currently must be at least 3 units,
we can assume a minimum of 3 units to be joined when the expected
count from `goal-state` is 1.
Also replace os-testr with stestr.
Closes-Bug: #1976537
Change-Id: I5b91d3caa466383fec76a393556668eb3b59ec63