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: Update Changelog