Add a volume driver in Nova 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: I84bf268e5a2c5c33b8706830e8067914fae44aed
Implements: blueprint scality-volume-driver
cfg should be imported from oslo.config

Change-Id: Idd9c3ededd2f63156eb78ed01fd38ddb3a20437f
Merge "Add a volume driver in Nova for Scality SOFS"
Merge "cfg should be imported from oslo.config"