Add an option to stestr run to not store in the repository

[mtreinish]

There are certain instances where you want to run tests in parallel but not store the results in the repository. We should have a flag on stestr run where we can say run the tests but don't track the results.

This is another feature motivated by the stestr run --analyze-isolation flag. We're inadvertently storing a lot of intermediate runs in the repository when during it's operation and it's not necessarily something we want to track. For the isolation flag whether we decide to leverage this feature or the --combine flag to store all the runs as a single entry doesn't change this would be a good feature to have.