qa/workunits/rbd: avoid unnecessary sleeping in stop_mirror()

qa/workunits/rbd: avoid unnecessary sleeping in stop_mirror()

There is no need to wait for anything if -KILL is passed for sig
because the process would disappear immediately.  In teuthology runs
where multiple rbd-mirror daemons are deployed (and therefore need to
be stopped when stop_mirrors() is called by the test), it causes
gratuitous delays of 4+ seconds.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit f18fe5d15f72e34ab74b8ae187a47b52883fc780)