This adopts https://pagure.io/quick-fedora-mirror/pull-request/27
and adapts to it, so we get `imagelist` files rather than
`filterlist` files (see recent commits for this). The rationale
is more fully explained in that PR (and in PR #26 also) - on
further inspection it turns out that we have to filter out an
awful lot of extensions to create small filterlists for all
three modules, and I'm worried that other file extensions may
appear in the future and cause the filterlists to suddenly get
bigger again. Instead, we have create-filelist use the productmd
constant that defines valid image formats, and only include files
that match those formats in the list. The downside of this
approach is we have to ensure productmd on all the systems that
run `create-filelist` is kept up to date if the list of valid
image formats changes.
2e09ca2d00