Add NMap tool for getting IP.

In some condition, vm may have fixed IP, but tcpdump couldn't get it until some packaged sent.
So can we start another tool 'nmap' parallel when tcpdump is working?
Following codes are simple description of this tool, just for hints. :)  

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
I'm not quite understand this ```if..elif...``` block
BTW, seems func ```scan_port()``` has the same part of this func