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.

Change-Id: Ie8894a7cebf350dc2d50567058a765482b9d97d7
Closes-bug: #1322366
Merge "Allow host config to be overriden in backend"