Be able to run scenarios from one feature file in different jobs

[joakimk]

We have some feature files in our project that takes up to 1.5 minute to run. Some can be split into smaller files but it would be practical if testbot could split them up into separate jobs that can be run in parallel.

The main benefit of this is that the overall test runtime can be reduced.

There is a branch for this: features_by_scenario.

This requires better balancing code (issue #5).