Global config should only support `local-context` on a per kube-context basis

@russwolf somehow had his ~/.skaffold/config sert to make all cluster be non-local which led to very puzzling errors when attempting to develop against local clusters. He’s unsure how this setting was configured. And this particular setting seems of questionable value as a blanket setting, and should instead be set on a per-kube-context basis.

We should revisit the actual global config definition as it allows invalid and questionable configuration combinations. For example, it allows setting insecure registries and debug helper locations on a per-kube-context basis.