When installing packages listed in control.yml, it may be desirable to disregard local config changes and force the installation of upgrades. Adds the ability to do this via config option, with the default being disabled.
* Adds "distro" as a supported install source for jenkins-job-builder.
JJB hit Ubuntu in raring. It should be possible to install this without the painful offline bundling if charm is deployed to a recent enough Ubuntu.
Also avoids an import error in gerrit helper by ensuring paramiko is installed.
NOTE: Leaving default for jjb-install-source to install from git as we are still installing on precise and 'distro' is >= 13.04 only so leaving default as precise-compatible.