Adds Windows Support to satori discovery

This change adds the capability to satori to do data plane discovery on Windows devices.
For that, a class has been added that mirrors the functionality of satori/ssh.py and
utilizes a "3rd party" script (satori/contrib/psexec.py) which is called via subprocess.Popen().
Further to that, an SSH tunneling class has been put in place that uses paramiko to establish
a tunnel (similar to running ssh -L from a shell). requirements.txt has been extended to
include impacket which satori/contrib/psexec.py imports.

Change-Id: I7a94eea9446bc7f57843407fb98880222f7af6af
Implements: blueprint windows-support