Revert "Add Scality SRB driver"

Revert "Add Scality SRB driver"

The Scality SRB kernel-driver is being re-designed to optimize for
specific workloads, which are (initially) not compatible with VM-style
block device access. Not to confuse users, we believe it's advisable to
remove the Cinder SRB driver in the meantime.

This reverts commit a23f17f8cebe5e1e57f675aedf6272257257d1b7.

Some references to the SRB driver and test modules added in later
commits are removed as well.

Conflicts:
        cinder/tests/unit/test_srb.py
        cinder/volume/drivers/srb.py
        etc/cinder/rootwrap.d/volume.filters

See: 2f9e4163f42ae5246fd997b9f35e16d3d97be54f
See: 3fda737f53824170bd59eb6e8ce2e991c89c3c1d

Change-Id: Ic9d79b09363ef904932128f63371ba01a15c5d31