Comment 13 for bug 1240667

Added the apt preferences file, as suggested in #9, also mongodb-server is installed with explicit --target-release 'precise-updates/cloud-tools' argument, so it's always the correct version. Tested on EC2 by bootstrapping and deploying openstack-dashboard on machine 0. It went fine and I was able to open the /horizon login page. No errors during charm installation, so I can assume it fixes the issue.