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.
snf_django: Fix unicode error in proxy requests

Pithos and Cyclades services need to proxy Astakos API calls for enabling
the web clients to access them.

Proxy requests containing non ASCII characters used to fail.

The reason was that the proxy tried to make a request using the original
request path.
However Django forces the request path to be unicode.
Therefore, the httplib used to fail because it tried to concatenate unicode
and bytestrings.

This fix changes the original implementation and from now on it sends
the requested URL encoded in UTF-8.
docs: Update install guides
deploy: Update node configuration

Make snf-deploy use the configuration for Wheezy instead of Squeeze.
changelog: Add CentOS documentation
ui: Include 'gateway_ip' in subnet create API call

Closes #67
pithos: Helper functions

Add helper functions for selectable storage backend.
pithos: Make blocker use selectable storage backend
pithos: Make mapper use selectable storage backend
pithos: Propagate selectable storage backend

Propagate selectable storage backend to blocker and mapper.
pithos: Introduce new variables

Introduce two new variables, PITHOS_BACKEND_STORAGE and
PITHOS_RADOS_CEPH_CONF.

Possible values for PITHOS_BACKEND_STORAGE are 'nfs' and
'rados'.

Remove unused variable PITHOS_RADOS_STORAGE.
pithos: Extend ModularBackend class

Extend ModularBackend class for the selectable storage backend.
pithos: Extend PithosBackendPool class

Extend PithosBackendPool class for the selectable storage backend.
pithos: Remove hardcoded Ceph conf file
pithos: Import two new configuration variables

Import two new variables for instatiating ModularBackend,
BACKEND_STORAGE and RADOS_CEPH_CONF.