* services: always set deregister flag after deregistration of group
This PR fixes a bug where the group service hook's deregister flag was
not set in some cases, causing the hook to attempt deregistrations twice
during job updates (alloc replacement).
In the tests ... we used to assert on the wrong behvior (remove twice) which
has now been corrected to assert we remove only once.
This bug was "silent" in the Consul provider world because the error logs for
double deregistration only show up in Consul logs; with the Nomad provider the
error logs are in the Nomad agent logs.
* services: cleanup group service hook tests
