Support Cinder scheduler hints

When creating volumes with Cinder, passing scheduler hints can be
necessary to select an appropriate back-end.  This spec proposes to
add a 'scheduler_hints' option for OS::Cinder::Volume objects, as is
it already done for OS::Nova::Server.

Change-Id: Ibc9055b5e37398dd29a807cace1d4235b85d4b30