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.
pithos: Instantiate PithosBackendPool class

Instantiate PithosBackendPool class with the appropriate
variables for the selectable storage backend.
pithos: Retrieve backend storage type from params
pithos: pep8 fixes
pithos: Add synchronization tool

Add 'pithos-sync-rados.sh' shell script in 'contrib' directory,
a Pithos+ object synchronization tool between NFS and RADOS.
pithos: Update admin guide
pithos: Update Changelog