cephadm: update _rm_cluster to call terminate_service

cephadm: update _rm_cluster to call terminate_service

The terminate_service function was added to encapsulate the act of
terminating a systemd service. The rm-deamon handler was updated to use
this function previously; this commit updates rm-cluster function(s)
to do the same. This needed a bunch of test updates do to how the tests
were mocking commands.

Signed-off-by: John Mulligan <jmulligan@redhat.com>