astakos: Improve exposed stats

Until now, Astakos exposed statistics per authentication provider.
However, since a user can have more than one authentication providers,
the administrator cannot sum the values per provider in order to compute
the total values. This commit fixes this issue by extending the Astakos
admin API to expose statistics for all users and resources.

Also, this commit updates the resources per provider response, to
include only the resources that are allocated to users that are using
only this provider.
cyclades: Refactor statistics mechanism

Major refactoring to Cyclades statistics mechanism. The admin API
endpoint has been updated in order to expose detail statistics about
Ganeti cluster, virtual servers and networks, public IPv4 pools and
images.

The 'stats-cyclades' management command has also been updated in order
to display all available statistics in a user-friendly way.
docs: Add guide about statistics API

Add a description of the statistics that are exposed by the admin API
of Cyclades and Astakos service.
Update Changelog

Add section in Changelog about unreleased version 0.15.1.