When operator has not enabled 'file' store and using other stores like ceph,
swift etc. the uploading to staging area works as we explicitly
build 'file' store during this operation, while cleaning up we directly
use 'glance_store.delete_from_backend' which only works if 'file'
store is enabled.
Modified '_DeleteFromFS' task and _unstage call which will use os
module to unlink the file present in staging area explicitly to
delete the data from staging area.
Closes-Bug: #1803498
Change-Id: If0b3b0af9300301291758c67267890e0959ebb3c
c927246085