Delete volumes after each scenario unless tagged @keep_volumes.

This makes scenario dependencies from volumes shared between them
pretty explicit (if scenario i is tagged, then scenario i + 1 depends
on scenario i), so we can remove the comment about that. It also helps
keeping the free disk space requirement down as volumes don't pile up
unnecessarily after they're not needed any more.
Finish backporting test/keep-volumes-tag to pre-test/reorg.

This will need to be reverted when we merge test/reorg.