Support beakerlib tests as prepare scripts

[lukaszachy]

While tmt supports ansible and shell as prepare plugins there are a lot of "tests" written in beakerlib which are setup/preparation steps in reality. If tmt could execute tmt tests as part of the prepare step it would be great.

It could be very similar to discover:fmf (and called prepare:fmf) Needs to support order, require/recommend, libraries, remote repo fetching. IMHO worth of new plugin to keep how:shell simple as it is now.