Allow check_for_* functions to be consumed by wait_for

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