Add pithos backend version check

Synnefo version 0.15.1 has introduced a selectable storage backend
for Pithos, 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.
Bump version to 0.15rc2

Update Changelog and version files.
Merge branch 'release-0.15' into debian-release-0.15
Bump version to 0.15~rc2-1~wheezy

Signed-off-by: Nikos Skalkotos <skalkoto@grnet.gr>