Cleanup and test volume usage on volume detach

* Refactor _poll_volume_usage to be more readable

* Test that detaching a volume updates the totals on the volume usage
  cache correctly. This works but adding test to make sure that it always
  works.

Change-Id: I82da44137bc65d5f04bca0820eb327438d6e1f35
Delete extra space to api/volumes message

Change-Id: I04efa9c8ddb5790f99d66c302198bf92000f6e66
Signed-off-by: You Ji <jiyou09@gmail.com>
Merge "Delete extra space to api/volumes message"
Merge "Cleanup and test volume usage on volume detach"