In order for the `check_for_*` functions to be consumed by `wait_for`,
they should notify of their success but not exit.
As a consequence, the previous behavior is restored by the fail_unless_*
companion functions.
With this change, it is now possible to do:
wait_for 30 1 check_for_os_service_running keystone
Change-Id: I16ddf8913027030c3ccb5487713d172904508fd6
34872c1c2f