qa/workunits/mon/test_mon_config_key: use subprocess.run() instead of proc.communicate()

qa/workunits/mon/test_mon_config_key: use subprocess.run() instead of proc.communicate()

the loop of proc.communicate() on python3.6, where we always are able to
get something out of stdout and/or stderr PIPEs. and the `stdout` and
`stderr` keep growing until out of memory. and teuthology considers
the command crashed after a while.

Fixes: https://tracker.ceph.com/issues/50393
Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit f2e0770c276e6fffa01a0bcb2d969f02ee1aafef)