Support rewriting the image name

So that I can drop [this hack](https://github.com/galaxyproject/pulsar/commit/79d5f23b81b1acbd1ae2acf681032a4a1b2e3cd6)

I am not sure, I think I tried an unstructured path rewrite for this and it didn't work, but I can check again, just didn't want to lose track of this issue.

Start by reading the linked commit 79d5f23b81b1acbd1ae2acf681032a4a1b2e3cd6 and trace how image names are currently handled. Confirm what kind of rewrite is required and whether the earlier unstructured path rewrite attempt applies. Done means supporting image-name rewriting so the referenced hack can be removed.

Written by the indexing model from the issue text.