[feature] Warn on kube-system installations

[maxlemieux]

Summary

kube-diag should warn if it sees that New Relic is installed to kube-system namespace. This is an antipattern - many direct references on internet including our blog post: https://newrelic.com/blog/how-to-relic/how-to-organize-kubernetes-clusters

Desired Behavior

Print a single warning line if kube-diag detects it is running against kube-system namespace (or if kube-diag autodetects that the chart was installed there).

Possible Solution

Parse namespace name.

Additional context

Beyond difficulty in maintenance of New Relic chart in system namespace, Installations here may experience unknown problems if cluster has special security on this system namespace.