Server test driver should use pgrep instead of jps

[OK:     0, KO:     0, SKIP:     0] Test starting: ResilienceIT.testUnresponsiveNode
[0] PAUSE Container.ExecResult(exitCode=0, stdout=Paused server with PID=
, stderr=/opt/infinispan/bin/pause.sh: line 2: jps: command not found

kill: usage: kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill -l [sigspec]
)
]]></system-out>