docs: Change quick install admin guide for debian

* Prepare quick install admin guide for the changes that will come with
the CentOS guide
docs: Add centos install guide

* Change the name of quick install admin guide to install guide debian
* Add CentOS installation guide (install-guide-centos)
* Update the doc index to comply with the aforementioned changes
* Update all pages that linked to the old guide
* Update both guides to be in sync with Synnefo 0.15
* Fix minor `bugs` in both Debian and CentOS install guides
* Add sections on how to create vlans/bridges/ssh-keys
* Update architecture images
common: Update default emails used in settings

* Update the default emails used in settings to point to example.com
instead of synnefo.org
cyclades: Minor bug fix in server create

* Fix a typo in logic/servers on server port creation
* Update the copyright date
snf_django: Fix unicode error

Fix unicode error in proxied requests
astakos: Fix TypeError in weblogin
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.