Add a volume driver in Cinder for Scality SOFS

Scality SOFS is a network filesystem mounted with FUSE, with most
options given in a configuration file. Given a mount point and a SOFS
configuration file as driver options, the Scality volume driver mounts
SOFS, and then creates, accesses and deletes volumes as regular
(sparse) files on SOFS.

Change-Id: I914714e8547a505109514e2072f9e258abca8bd4
Implements: blueprint scality-volume-driver
Merge "Add a volume driver in Cinder for Scality SOFS"