Consider deploying Netchecker

[NicolasT]

We currently don't deploy Netchecker (which is optional in Kubespray). We should consider doing this, though, and integrate it with Prometheus (both capturing metrics as well as having some alert rules).

We could deploy it using the Kubespray support for it, or inside MetalK8s.

https://github.com/Mirantis/k8s-netchecker-server/
https://github.com/Mirantis/k8s-netchecker-server/blob/master/doc/metrics.md
kubernetes-sigs/kubespray#2701

[thomasdanan]

@gdemonet removed legacy label and linked this issue to Network Healthchecks EPIC