Introduce new Pithos variables

Introduce new Pithos variables for selectable storage backend.
Extend Ganeti create script

Include new Pithos variables for selectable storage backend to Ganeti
create script.
Extend pithcat for selectable storage
Add default values for Pithos backend and RADOS

Set default values for Pithos storage backend type, RADOS conf file,
RADOS map and block pools.
Update documentation