Provide a default for OPENSTACK_IMAGE_BACKEND

The list of image formats has become customisable, but didn't have a
default which can cause issues since it's a required field when
creating a new image. Also document the setting.

Additionally, mark OpenStack settings documentation as "(Partial)"
until #1221115 is closed.

Change-Id: I980c2d61bc9f9ac14fa39e5088c2ce03c48499b2
Closes-Bug: #1223427
Merge "Provide a default for OPENSTACK_IMAGE_BACKEND"