Ticket 47613 - Issues setting allowed mechanisms
    
    Bug Description:  Adding an empty value for nsslapd-allowed-sasl-mechanisms blocks all
                      sasl authentication.  Also changing the allowed sasl mechansism does
                      require a restart after making a change.
    
    Fix Description:  Reject an empty values for nsslapd-allowed-sasl-mechanisms, and allow
                      config changes to occur without restarting the server.
    
    https://fedorahosted.org/389/ticket/47613
    
    Reviewed by: nhosoi(Thanks!)
    (cherry picked from commit 43959232f792db2b79e614f6db78f7569920fdc1)
    (cherry picked from commit a1e386188663c9197b80b3b51cca0d58ce0c9181)