Code review comment for lp://staging/~al-maisan/launchpad/builder-data-502841

Hello there!

This is another piece required to get us to the point where we can estimate
job dispatch times in the Soyuz build farm irrespective of job type.

The code added

  - retrieves data about the builders that are available in the system for
    the various processor architectures and virtualization settings.
  - check how many builders are free/available to take on a job for a given
    build (processor and virtualization setting).

Tests to run:

    bin/test -vv -t TestBuilderData

No "make lint" errors or warnings.