Make Image Service image formats configurable

Allow configuration of the available image formats
for image creation (corresponds to a feature added
Glance).

Incidentally fixes a test issue where the domain_get
call wasn't being properly stubbed out.

Fixes bug 1216157

Change-Id: Ifc72915d34767ce83f9d85b92ddeae0c2c00b8c6
Merge "Make Image Service image formats configurable"