provide repository-clone list as text response

If the repository list is queried as text/plain it will respond with the
clone-urls for the repositories.  One can switch between the different
clone-urls via `?clone_type`, where `?clone_type=git` is the default and
`?clone_type=ssh-git` is currently the only alternative.
extract default-clone-type into constant

The default-clone-type of 'git' is now a constant in the
Repository-namespace. As this is the default clone-type the
clone_methods method will now use this as the default (instead of the
confusing `nil`).
add link to repository clone list.
Merge pull request #1083 from ontohub/1082-provide_url_list

1082 provide url list