Add warning about only one existing CA server
    
    It is not safe to have only one CA server in topology. Therefore there is a check
    and in case that there is only one CA server a warning is shown. The warning is
    shown after each refreshing of servers facet.
    
    https://fedorahosted.org/freeipa/ticket/5828
    
    Reviewed-By: Tomas Krizek <tkrizek@redhat.com>