Allow 'nfs:NONE' in global configuration
    
    This patch adds 'nfs:NONE' as an allowed entry for the global
    authorization data type in the CLI and WebUI. This is an ad-hoc solution
    to make sure that the new default value for the NFS service is not
    removed by chance.
    
    This patch should be removed if a more generic solution is implemented
    to modify service:TYPE style values of the authorization data type.
    
    https://fedorahosted.org/freeipa/ticket/2960