stbt batch run: Make saving video optional; with default True

Until we find a solution for NoVideo exceptions during STB recovery, we
need a workaround in order not to block memory tests that don't require
triaging the results (the only point is to keep the boxes under load).

This commit makes `--save-video video.webm` the default option; you have
to specify `--save-video ''` in order not to save video.