Option type.
'ascii' means using the ascii string that is defined by
hh3cDhcpServer2OptionAscii to fill in the option.
'hex' means using the hex string that is defined by
hh3cDhcpServer2OptionHexStr to fill in the option.
'ip' means using the IP string that is defined by
hh3cDhcpServer2OptionIPStr to fill in the option.