Setup AppVeyor CI for automated testing on Windows

[lmakarov]

AppVeyor will officially support Windows workers with nested virtualization support soon. This will allow us test Docksal on Windows in a CI environment.

AppVeyor is free for open-source projects, however, Premium workers (with nested virtualization / Docker for Windows support) are will be offered at a 50% discount for OSS. We may need to find a sponsor to cover the cost of CI and automated tests for Windows.

• https://help.appveyor.com/discussions/questions/2652-windows-server-2016#comment_42637798
• https://ci.appveyor.com/project/appveyor-tests/docker-ce

[lmakarov]

AppVeyor has released the On-premise Edition with a free tier: https://www.appveyor.com/on-premise/

There would still be the cost of running the actual Windows VM.

A rough comparison of hosted vs on-prem costs for our CI needs:

• Hosted: ($29/month for basic plan + $100/1 job/month for premium worker) * 50% off for FOSS = $65/mo
• On-prem: free AppVeyor Community Edition + unknown for a Windows VM on Azure (or free if using an existing machine somewhere) = free - 100$ (?)

Going with hosted would be more reliable. Going with on-prem could be almost free.