Save and report nodepool image names

When building an image, save the hostname of the image to a file in
/etc.  Before running tox in jobs, cat that file.

This will help confirm which image was used to build a given node for
a particular job, and potentially help debug some issues we are
currently seeing.

Change-Id: I9d55b7223ea85c1137b460f049a0184d7e9ce53d
Merge "Save and report nodepool image names"