remove usage of flexvolume in kubeadm

[neolit123]

distroless effort:
kubernetes/kubernetes#70249

see this message from sig-storage https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/kubernetes-dev/zC8jCLg142w/2P3BN5oTAgAJ

We are gauging the viability of deprecating the Flexvolume master API calls.
...
There is an effort underway to move to core k8s component images to distroless.

kube-apiserver and kube-scheduler already moved to distroless, but the kube-controller-manager was blocked due to flexvolume. kube-proxy is yet to move too.

currently kubeadm has related logic to manage flex volumes for the kube-controller-manager static Pod:

  - hostPath:
      path: /usr/libexec/kubernetes/kubelet-plugins/volume/exec
      type: DirectoryOrCreate
    name: flexvolume-dir

    - mountPath: /usr/libexec/kubernetes/kubelet-plugins/volume/exec
      name: flexvolume-dir

IIRC the above is currently GA and required for the KCM to run properly.
in case the KCM / kubelet deprecates and remove flexvoluime support kubeadm should follow.

upstream ticket:
kubernetes/kubernetes#98815

[xlgao-zju]

@neolit123 Since kubernetes/kubernetes#91329 has been merged, shall we remove the mount of flexvolume-dir now?

[neolit123]

so my understanding is that flex volume support does not work with the distroless image for the KCM that k8s ships by default now. however, someone might decide to build their own image and override the one that kubeadm uses from k8s.gcr.io.

this leads me to believe that instead of removing the kubeadm support for flex volume today, we should wait until flex volume is completely removed.

cc @dims please correct me if needed on this one.
cc @rosti

[dims]

yep well said @neolit123

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[neolit123]

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