Add VPN interference check: HTTP fetch before installing Kubernetes

[tstromberg]

Here's an idea to address #4302

Add a lightweight HTTP service to our VM that unobtrusively reports system stats, such as load average. As soon as we can confirm SSH access to our VM, and the VM appears to be of the correct version, try accessing this service.

Port 8443 would be ideal for catching the issue, but lets say a nearby port to avoid conflicts.

This would kill two birds with one stone:

• A lightweight way to gather system metrics without impacting VM load (compared to ssh+uptime command)
• A way to detect connectivity issues separate from Kubernetes.

If it fails, we could report a very clear message, such as:

Unable to make HTTP requests to the VM (http://<ip>:port). To use minikube, please adjust your firewall or VPN to allow these connections.

We can always add --force to workaround issues with it.