Ticket 47910 - logconv.pl - validate start and end time args
    
    Description:  Add validatation checks for the startTime/endTime configuration
                  arguments(-S, -E)
    
    https://fedorahosted.org/389/ticket/47910
    
    Reviewed by: rmeggins(Thanks!)
    
    (cherry picked from commit 8495afa57ad837e3a51871a4f6da2a9978c8e711)