Add cinder volume driver support for Nimble Storage.

The set of changes introduces the cinder iSCSI volume driver support for
Nimble Storage. It implements the minimum set of features required by the
icehouse release. Please see the blueprint nimble-iscsi-volume-driver for
more information.

Change-Id: I48781fc890b8fefb33027da3db2b1ff56c5deed4
Implements: blueprint nimble-iscsi-volume-driver
Merge "Add cinder volume driver support for Nimble Storage."