Add instructions for the CLI usage for Windows users in the lab's README.md

[Matjaz Pancur]

New Windows users don't have a desctiption in the labs' README.md file about how to enable CLI usage. They can't run admin-openstackrc.sh or demo-openstackrc.sh directly.

Proposed solution (from https://review.openstack.org/#/c/150335/)

For Windows, we need to write a separate version of *rc files. There are three options:
1. we just write in the README, that these env variables need to be set somehow (manually in "Environment variables" GUI dialog or that Win users can use proper switches --os-username, etc. with every command)
2. We include a Windows Powershell version of our rc files (That's what Cern is doing, look at their Horizon patch https://github.com/cernops/openstack-cern-patches/blob/master/python-django-horizon/1004-cern-python-django-horizon-windows-powershell.patch)
3. We include a "generic" Powershell wrapper script around our rc files (e.g. look at https://github.com/naturalis/openstack-docs/wiki/Howto:-Installing-and-configuring-the-OpenStack-commandline-tools-on-Windows)

#1 is too hard for new Win users, so #2 is the easiest way, #3 could be too complex and error-prone.

[Roger Luethi (rl-o)]

I expected that Windows users would do something else:

0. Boot the VirtualBox VMs with a GUI (console), log into the controller VM console, and use the CLI tools from there.

All the other options expect the users to install/configure additional software (e.g., ssh for Windows). I agree that option #2 would be nice to have.

Also, I think we should have a README_WINDOWS.rst, because pretty much everything is different on that platform.

[Matjaz Pancur (matjazp)]

Roger, I agree that using client tools on the controller is the easiest way and the way that trainees will use during Meetup training sessions, because it has the added benefit of a unified and predictable environment. When you focus on the basics, that makes sense.

However, having a way to show Windows users that they can natively use OpenStack CLI has numerous benefits: it shows that OpenStack can be usable and controllable from all platforms, not just Linux/OSX. That is important in the environment where Windows are still the dominant platform on clients (read: almost all laptops except Macs... looking at my students laptops only a few run Linux on them, and we're talking about CompSci students. Same goes for Enterprise environments).

[Roger Luethi (rl-o)]

Good points, and interesting for me to hear. I am glad Pranav took this bug (although I think marking it "critical" is somewhat overstating the severity of this bug).