Allow host config to be overriden in backend
Allow host config to be overriden in backend

This allows you to specify host=XXX in your backend configuration
to override the host value that is specified in the DEFAULT section,
allowing for some backends to share a hostname while others can
have unique names.

Closes-bug: #1322366
DocImpact: When using multi-backend, each driver can  now specify
           'host' parameter.
(cherry picked from commit c0630419f45f0b34bae5702ccecedf0b398f91b0)
Change-Id: Ie8894a7cebf350dc2d50567058a765482b9d97d7

NeCTAR-RC/cinder,NeCTAR-RC/cinder