Add back more of the _get_runner() logic
Add back more of the _get_runner() logic

This commit adds back some of the previously removed logic from
_get_runner() the bisect tests are failing on windows py36, so it's
likely some of the fallback logic that was removed was actually
necessary for that environment. I'm not able to test this environment
locally so hopefully this is what was missing.

masayukig/stestr,mtreinish/stestr,masayukig/stestr,mtreinish/stestr