Code review comment for lp://staging/~ev/daisy/core-storage-providers

Looks great really, tnx for it.

A fix , and a couple of nits (as per IRC chatter)
#1 we need to join provider_data['path'] also at retracer.py
#2 lets s/nfs/local/ everywhere, likewise s/write_to_san/write_to_local/
#3 please decouple the new CF usage ('Bucket') to another MP, ie let's keep using 'Buckets' meanwhile