This adds an hybrid storage backend to CloudKitty. It aims at splitting the
processor state management and the dataframe storage part. The default backend
for this storage is gnocchi, but other backends will be implemented in the
future.
This storage backend is a standard storage module for now, but it will become
the default storage once it is considered stable. Once this new storage type
has become the default, storage backend creation will be easier (no state
handling).
Change-Id: I61c0c17230350b12be3484ea4b5805960aa33099
Story: 2001372
d2f7e06362