named.conf template: add modification warning
    
    Warn the user some modifications may break IPA setup or upgrade.
    All changes the user makes should be explicitly supported and mentioned
    in the documentation. Undocumented and unsupported changes, such as
    renaming dyndb part from "ipa" will break IPA.
    
    Signed-off-by: Tomas Krizek <tkrizek@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>