pithos: Remove prefix for Archipelago volumes

Introduce new property 'is_snapshot' and 'is_available' and
remove old 'archip' prefix for Archipelago volumes.
Refactor relevant functions.
pithos: Extend ObjectWrapper class

Add metadata to Objectwrapper initialization variables and
propagate 'is_snapshot' property to backend functions.
pithos: Propagate 'is_snapshot' property

Propagate 'is_snapshot' property to modular backend functions.
pithos: Suppress value from modular backend

Suppress 'is_snapshot' return value from modular backend
get_object_hashmap function.
deploy: Update Gunicorn startup conf file
plankton: Pass conf values to Pithos backend

Pass Archipelago config file, XSEG pool size and
mapfile check interval configuration values when
instantiating Pithos backend class.
pithos: Change mapfile check interval default value

Set default value to 1sec instead of 5sec. After taking
a snapshot the user may try immediately to create a new
machine based on the last snapshot. If the mapfile check
interval value is high enough then the end user has to wait
at least the defined interval.
pithos: Expose Archipelago pool to modular backend
pithos: Slight modifications in the backends

* Do not generate/register a mapfile for zero size objects since
there are not stored in the storage backend.
* Several minor fixes
pithos: Fix/improve pithos app tests
pithos: Skip test in case of sqlite backend module
pithos: Remove unused block path and umask variables
plankton: Remove Pithos unused block path variable
pithos: Update backend tests
pithos: Fail fast while trying to copy an unavailable snapshot.

Check whether an object is available in the storage backend before copying it.
Otherwise, fail with 403 (FORBIDDEN) response status.
docs: Update Object Storage API