Add downloads.theforeman.org vhost

This will be a new location for tarballs (which we can pull into packages) to replace redmine (http://projects.theforeman.org/projects/foreman/files) which has unpredictable URLs and is harder to upload files to automatically.

It can also serve as a better place for discovery images instead of using {deb,yum}.tf.org.

@ohadlevy would you mind adding this to DNS please?