Validate and convert certificate SN
    
    The cert plugin only worked OK with decimal certificate serial numbers.
    This patch allows specifying the serial number in hexadecimal, too. The
    conversion now works such that:
     * with no explicit radix, a best-effort conversion is done using int(str,
       0) in python. If the format is ambiguous, decimal takes precedence.
     * a hexadecimal radix can be specified explicitly with the traditional
       0x prefix
    
    https://fedorahosted.org/freeipa/ticket/958
    https://fedorahosted.org/freeipa/ticket/953