Ensure add-apt-repository exists

With trusty, when installed via PXE, it's possible to end up
without add-apt-repository in the target.  This ensures it is
available.  Unfortunately, it is provided by different packages
across different distributions.
Clean up kernel arguments passed to stage2

Remove support for installing distros older than Precise via PXE,
and standardize the kernel arguments.  For anything we don't know
about, ask the user rather than guessing wrong.  This still allows
the installation of ancient distros.