make yum-repo-server-repos dnf compatible

[pluralistix]

As DNF (http://dnf.baseurl.org/) uses the repo-xmls instead of the repo-sqlite and DNF becomes the new default package manager starting with Fedora 22 (Release date 26.05.2015) those clients won't be able to use repos, which are created and maintend by yum-repo-server. Furthermore - clients having such repos attached will not be able to refresh metadata for all repos, as an refresh/download will fail completly. Please introduce a switch which allows to decide, which kind (xml, sqlite or both) of metadata is generated.

[heroldus]

In our case, repo-xmls will be huge and the generation will take a lot of resources. This really takes time in implementing and tuning...