Despite the image_file parameter not being configurable anymore,
image_file is still an internal attribute from the UrlGlanceImageFixture
class, which depends on the configurable image_url parameter,
whose value could be either an http(s):// URL or a file:// URL.
Hence, local stored images can still be used by setting image_url to
something like this: file:///tmp/images/myimage.qcow2
Change-Id: Iee77359037ecab4536ea3e904a7d659fbf2aa7b0
22503c28a2