tests: prevent kill race condition

When trying to kill a daemon, keep its pid in a variable instead of
retrieving it from the pidfile multiple times. It prevents the following
race condition:

  * try to kill ceph-mon
  * ceph-mon is in the process of dying and removed its pidfile
  * try to kill ceph-mon fails because the pidfile is not found
  * another ceph-mon is spawned and fails to bind the port
    because the previous ceph-mon is still holding it

Signed-off-by: Loic Dachary <loic@dachary.org>