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>
Yes, it just looks like the scan_port.
My original intent is to use scan_port for dealing mac/ip.
But then I found for wait_for_login(), I have to keep a thread to nmap ips with a timeout.
So this handler is used for start_nmap(timeout=xxx), while scan_port is one-off.