Infer the 'image' URL correctly when inserting a server.

When inserting (creating) a new server, using a private (project
specific) image, the code would infer the resource URI for the
image incorrectly, because the 'resource_url' on the image model
always returned a path to the 'global' pool. This patch reverts
the code to its form in the 1.12 release, which was able to work
with non-global images.

Refs #1933