[r=dimitern],[bug=1240667] Fixed bug #1240667: pin priority for cloud-tools
This introduces several new calls in cloudinit, which deal with apt sources, their preferences (/etc/apt/preferences.d/) and packages that we install from there.
Basically, when deploying a precise machine that needs to install mongodb-server package, we add the cloud-tools pocket, but by default this will add it as an apt source with a higher priority than the main archive. As a consequence, charms that try to install packages from main, which are also in the cloud-tools pocket get the latter, rather than the former (i.e. the described problem with python-django's version 1.5 in cloud-tools vs. 1.14 in main, which breaks openstack-dashboard charm).
Now, when adding the cloud-tools archive we also change its apt preferences, so that its priority as an apt source is lower than main, which means when installing mongodb-server during cloudinit we need to explicitly specity --target-release 'precise-updates/cloud-tools' to pick the version from cloud-tools.