image builder should check if master PPA has previously published packages

If no packages were published to a PPA, it won't have an index and will fail apt-get update. To avoid image builder to break because of that, we need to add a check to see if any packages were published to the PPA, and only add it to sources.list if so.

The workaround for now is to guarantee a package was previously published in the PPA.

The fix should be something around this:
>>> ppa = lp.me.getPPAByName(name="ppa")
>>> series = ppa.distribution.getSeries(name_or_version="precise")
>>> ppa.getPublishedSources(source_name='xpdf', distro_series=series))
[<source_package_publishing_history at https://api.launchpad.net/devel/~ursinha/+archive/ppa/+sourcepub/2616332>]