Clean sercomm_dump_config

Hi @mandreko,

This pull request tries to clean https://github.com/rapid7/metasploit-framework/pull/2822 but I need you to test it becuase I haven't a vulnerable device to test atm :( My mainly worry is if get_once on dump_configuration is going to work okey, that is, grabbing the full network data on the timeout, before proceeding with parsing. If it isn't working for you, pcap and module output is super welcome so hopefully I can finally fix it :)

Thanks in advance!