JCLOUDS-457: Add Clear vault strategy

Added a proper implementation to the Clear vault strategy
@rcoedo I pushed the first of these two commits to master.  I do not believe the second commit is correct; `clearContainer` should not delete a container.  Instead, perhaps `BlobStore.deleteContainer` should call `deleteVault` first, then call `clearContainer` if it fails, and finally call `deleteVault` again?