End ghost usage on sync

### End ghost usage on sync

We call "ghost (billing) resources" OpenStack resources that were deleted from OpenStack and are still (wrongly) charged by Fleio. Ghost billing resources appear when something went wrong, e.g. notification was not received from OpenStack, or it was wrongly processed by the "updated" Fleio container.

If you check "End ghost usage on sync", "fleio sync" script checks for ghost billing resources after syncing the list of OpenStack objects (e.g. volumes). To avoid concurrency issues, on the first sync run, they are marked as potential ghost billing resources, and on a subsequent run they are ended (the billing end timestamp is set to "now"). This means that you must run "fleio sync" twice to end billing ghost resources.