This adds a v2 storage interface as well as a gnocchi backend for it to
cloudkitty. This new interface will allow us to implement the v2 API, which will
be more metric-oriented.
This new storage uses the new dataframe format ('desc' is now split up into
'groupby' and 'metadata', following the collect logic); and supports grouping,
filters and pagination.
Modifications:
* The tenant/scope state management isn't handled by the storage backend
anymore. A StateManager object has been added.
* All internal interactions with the storage backend use the v2 interface.
An adapter class has been added to ensure retrocompatibility with the
v1 interface.
* A base class for functional tests testing the v2 storage interface has been
added. It contains generic tests, which should allow easier testing for new
v2 backends.
* Some developer documentation explaining how to implement a v2 storage
backend has been added. User documentation will be updated once the v2
storage backend is considered stable.
The v1 is still the default storage version. Note that this new version is
considered unstable and should only be used for upstream development.
Change-Id: I5e9b95c79292582fab3017289d35ee310e35ffea
Story: 2001372
Task: 6585
Task: 14361
Task: 24535
3ab4774f66