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
Only create volume with an active image

1. Currently cinder doesn't check whether the image is active
when create volume with an image id, we can even create volume
with a deleted/saving/queued image.

2. For deleted image, it only happened when using admin credentials.

When using non-admin credentials, it returned an error that image
is not found by using 'glance image-show' on a deleted image, which
is expected. Also when creating a volume from this deleted image,
it returned  a 404 error upfront. This again is the expected result.

When using admin credentials, we can view the details of deleted image
with 'glance image-show'. If create a volume out of an image ID (which
is now deleted), the operation succeeds, but the volume is left in
'error' state.

3. This accounts for a bug(he volume shouldn't be created) and should be fixed.

Change-Id: If4f59695fbf6636778d0d352aef28d90bb45ee91
Closes-Bug: #1302621
Merge "Allow host config to be overriden in backend"
Merge "Only create volume with an active image"