ksck master addresses argument is error prone

It's normal to pass -master_addresses=m1.example.com,m2.example.com when working with Kudu CLI tools. The ksck tool, however, expects the master addresses as a positional argument. This is fine, except that the error message returned when the -master_addresses flag is provided is terrible: Invalid argument: must provide master_addresses. It should either have a better error message, or ideally allow the argument to be passed as a flag (although this may be tricky with tools with more than one positional arg).