isolation option for slave and exit status of status_of_proc

That pull adding option to handle isolation variable for debian slaves and fixing fallowing bug:

root@job1(00:06:35):~$ /etc/init.d/mesos-slave status
[FAIL] mesos-slave is not running ... failed!
status_of_proc -p "/var/run/mesos-slave.pid" "/usr/local/sbin/mesos-slave" "mesos-slave"
root@job1(00:06:39):~$ echo $?
0