Make /etc/kubernetes/ a constant

[luxas]

The /etc/kubernetes/ path should be a constant in kubeadm and not changeable. We have to draw the line between configurability and standardization, and if you do want to use an other path (although there's no reason to), a lot of other things will break.

So myself and @mikedanese made the decision in #100 that this path should not be configurable, it's a standardized path for k8s deployments.

In the same manner, /etc/kubernetes/manifests is the standardized path for Static Pods.

Right now, /etc/kubernetes is configurable via an env param in kubeadm, which leads to extra complexity.
Instead, a constant should be made in cmd/kubeadm/app/constants and consumed everywhere.

[luxas]

I think this could be a good first contribution for @wilkers-steve and @larryrensing 👍

[wilkers-steve]

@luxas Sounds good! We'll start on this today

[fabriziopandini]

This topic is related also to kubeadm #45008 kubernetes/kubernetes#45008 addresses "Fix behaviour of parameters insensible to KUBE_KUBERNETES_DIR env var"

[pipejakob]

It looks like the const exists now, but is it still desirable to completely kill the ability to override it? #232 seems to be a valid usecase for wanting to write files elsewhere, if the user really knows what they're doing.

[luxas]

@pipejakob @errordeveloper How common is it to have /etc ro-mounted? Is it possible to ln -s in some way to workaround that while keeping /etc/kubernetes a constant in kubeadm.
I want to minimize complexity inside of the tool and I think having the manifests in different places may mostly lead to confusion... as noted in the design proposal

[luxas]

Now fixed