Add Scality SRB driver

This patch implements the `srb-driver` blueprint. It uses a similar
approach as Ceph's RBD in the way it exposes a block device to the
system.

The driver controls the SRB Linux kernel driver through a sysfs
interface and leverages LVM for snapshot management. The native block
driver talks to Scality's storage system (or any other vendor exposing
a compatible CDMI interface) through a HTTP-based RESTful protocol.

iSCSI export to Nova is provided.

This driver depends on the extensions to the LVM brick proposed in
https://review.openstack.org/#/c/138348/

Implements: blueprint srb-driver
See: https://blueprints.launchpad.net/cinder/+spec/srb-driver
Change-Id: Id8d00df9db4004d5aeb8c0269d114ef50e0b8f8e