Improvements to IBuilder.findBuildCandidate()

With the fix for bug 393546 multiple builds for the same PPA+arch will only happen in series (to avoid builder monopolization by nightly builds for multiple distroarchserieses).

We may want to improve this solution so that, for example, builds with a score above a certain threshold will always be found as possible build candidates. This will allow, for example, security ppa builds for the same arch to be built in parallel.

This could be implemented in IBuilder._findBuildCandidate() in the clause that already ensures multiple builds for the same ppa+architecture are not considered concurrently, but IMO will require creating a new test_builder.py unit test to properly test (without making the doc test more complicated and less readable than it already is).