Create /var/run/jenkins directory if it doesn't exist yet

Without this patch, my jenkins installations don't start up automatically after reboot. This is due to the fact that the  `/var/run/jenkins` directory is missing. I have to create it manually each time I reboot the servers. This PR fixes the problem.

The code is analogue to [this](http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/dev-db/mysql-init-scripts/files/mysql-5.1.67-init.d?revision=1.1&view=markup#l67), which is the init script for mysql in the portage tree itself.