update the MOSIX command

sshCluster->exec() was hanging with the latest version of phpseclib.

why?
nohup was printing some standard output to the console.
The older version of phpseclib does not hang if nohup prints standard output.
The latest version of phpseclib hangs if nohup prints standard output.

fix?
make sure nohup doesn't print anything to the console by adding: "&> /dev/null"