(v3) 	Add :cleanup_rollback task

This patch adds a new task `deploy:cleanup_rollback`,
- to archive a rolled-back release dir in tar.gz and store it in deploy path
- and then delete the rolled-back release dir.

Important: This branch is built on top of #559. So, you can either merge #559 before merging this, or ignore #559 and merge this PR only since it contains #559's commit.