pithcat: Add support for Archipelago images

Synnefo version 0.16.0 completely removes the selectable storage backend
for Pithos, and introduces an integration with Archipelago, resulting in
necessary changes to the ModularBackend class.
In order to keep backward compatibility with older Synnefo versions, we
check the Pithos backend version and instantiate the ModularBackend
class appropriately.
pithcat: Fix a bug when using Synnefo version 0.16

PITHOS_INPUT_DATA env variable and '--data' option are not used
in Synnefo version 0.16.
Set synnefo prefix