Make generic waitcondition for os-refresh-config

Most of the time we will find it useful to pass a wait condition in for
os-refresh-config to signal when it has completed successfully.

This supersedes the notcompute element's completion-handle, so we can
remove that element. It should be removed from tripleo-incubator and
tripleo-ci as well.

Change-Id: Iff65da3dd0a1db496cfd33b4a55abb13c68c9558