Fixes #8444 - Expose cdn_ssl_version as an installer

$ katello-installer --help | grep cdn-ssl

  --katello-cdn-ssl-version     SSL version used to communicate with the
CDN. Optional. Use SSLv23 or TLSv1 (default: nil)

$ katello-installer  --katello-cdn-ssl-version=TLSv1
will add the following to /etc/foreman/plugins/katello.yaml

common:
  cdn_ssl_version: TLSv1