pithos: Helper functions

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

Propagate selectable storage backend to blocker and mapper.
pithos: Extend ModularBackend class

Extend ModularBackend class for the selectable storage backend.

Introduce new variables, PITHOS_BACKEND_STORAGE and RADOS_CEPH_CONF.

Possible values for the PITHOS_BACKEND_STORAGE variable are 'nfs' and
'rados'.
pithos: Extend PithosBackendPool class

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

Introduce two new variables, PITHOS_BACKEND_STORAGE and PITHOS_RADOS_CEPH_CONF.
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: Use rados module only when using RADOS backend