This commit switches to the librepo enabled `images` library via:
```
go mod -replace github.com/osbuild/iamges=github.com/mvo5/images@librepo-sources-osbuild1974
```
which in turn needs osbuild PR#1974.
With that it then adds a new `--use-librepo` switch that will
enable librepo based downloading so that people can play with
the new backend.
This commit adds integration for the ostree options. It is modelled
loosely after weldr-client/composer-cli and the
```
start-ostree --{ref,parent,url}
```
and uses
```
--ostree-{ref,parent,url}
```
A simple smoke test is provided that uses fedora-iot. Ideas welcome
for an easier way :)