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
Handle maxclonepervolume/node limits in SF driver.

The SolidFire cluster has a max simultaneous clones per volume and node
limt that we weren't accounting for.  Add some logic to detect this situation
and loop until enough simultaneous clones are completed to allow processing
of the next one in line.

Fixes bug: 1129810

Change-Id: Id63c1b142a81df5d9d2c4e39fb821b03ebac63a2
Merge "Handle maxclonepervolume/node limits in SF driver."
Merge "Add a volume driver in Cinder for Scality SOFS"