Ticket 48916 - DNA Threshold set to 0 causes SIGFPE
    
    Bug Description:  If the DNA threshold was set to 0, a divide by zero would
    occur when requesting ranges.
    
    Fix Description:  Prevent the config from setting a value of 0 for dna threshold.
    
    If an existing site has a threshold of 0, we guard the divide operation, and
    return an operations error instead.
    
    https://fedorahosted.org/389/ticket/48916
    
    Author: wibrown
    
    Review by: nhosoi, mreynolds (Thank you!)