Looks like new_portmapping_description, new_external_port and new_internal_port are good candidates for instance variables. I've tried to address by myself on: https://github.com/m-1-k-3/metasploit-framework/pull/8

Feel free to review, discuss, test etc. and land once ready. I also have checked the pcap's and look good. If these changes aren't breaking nothing, once landed it will be mainly ready to go I guess :)

thanks!