[SPARK-4832][Deploy]some other processes might take the daemon pid

Some other processes might use the pid saved in pid file. In that case we should ignore it and launch daemons.

JIRA is down for maintenance. I will file one once it return.
I agree this looks like a better test, but isn't it redundant with `kill`? 
Or: in case you have detected that the PID file existed but isn't the expected process, log a message?
You could use `ps -p` for consistency in both cases.