Fixes Bug 1040292 - replace create with get, keeping create as fallback

whenever the Ceph crashstore wanted a bucket that it did not already have cached,it would call "create_bucket"  For a save operation, it ought to try to "get_bucket" first and on failure, then try create.
Hmm so won't this still create the bucket if it doesn't exist, in the ```fetch``` case? Doesn't that mean that from the UI someone could ask for phony crash IDs and cause us to create buckets for no good reason?