All drivers call (or at least should) the helper method
successful_completion method when they're driver action has
completed succesfully. Some tempest tests exposed an issue where
the load balancer statuses would get updated before children were,
causing an inconsistent behavior. The load balancer should always
have its status updated last.
Change-Id: I4e422b7feaec5110ee856994631785e6e55a467e
Closes-Bug: 1534886