Sleep before ELB tasks and free-form old_build_version

- Sleep before ELB tasks because it's really hard to check RightScale
  Multi Run Executable status
- Changed old_build_url to free-form old_build_version string, to
  make the RunDeck job more user-friendly
- Skip the delete tasks if old_build_version is not provided on the CLI

Change-Id: I77f4626e8fd71bb6469442e1c091f1ca55e74a21