Add the ability to run for (consider) only a selected subset of packages

[Łukasz Zemczak]

This is a feature request, not a bug.

Recently when discussing with other release team members, we found that it would be useful to be able to run britney only for a selected set of packages currently in the given proposed archive. As an example use case: the archive is frozen and we're in the middle of preparing a release. A new release critical fix is coming in which got accepted into -proposed fast. We know that we are currently only interested in this one particular package (or set of packages) for the milestone, so we force a run of britney only on the packages of interest. This means running all the britney stuff (installability, autopkgtests etc.) only for a small subset of the archive, hopefully making things slightly faster.