SERVER-14679 fixed init.d script to create directory for pid file

Startup scripts not fit for CentOS 7 / RHEL 7

Default location for pid file is under /var/run/mongodb. /var/run can be mounted
on a temporary filesystem so the pid file directory is
not guaranteed to persist across reboots.

Closes #723

Signed-off-by: Benety Goh <benety@mongodb.com>