Increase kernel.pid_max to a higher value

While running big boxes with 72 disks it's easy to get out of PID for
all the threads needed by Ceph. Increasing the default value removes
this limitation.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge pull request #138 from leseb/increase-pid-max

Increase kernel.pid_max to a higher value