Refs: #3543

* Substitute the PITHOS_BACKEND_QUOTA setting with two
distinct settings: PITHOS_BACKEND_ACCOUNT_QUOTA &
PITHOS_BACKEND_CONTAINER_QUOTA
* Set PITHOS_BACKEND_CONTAINER_QUOTA default value
to something big (50 TB)
* Pass account and container policies as backend
arguments arguments instead of setting them
after each backend instantiation.