Add support of sub_path_expr for container volume_mount

[benjah1]

Description

Add support of sub_path_expr for container volume_mount

subPathExpr was added the VolumeMount since 1.15 as beta. It because stable since 1.17.
https://v1-18.docs.kubernetes.io/docs/reference/generated/kubernetes-api/v1.18/#volumemount-v1-core
https://kubernetes.io/docs/concepts/storage/volumes/#using-subpath-with-expanded-environment-variables

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[benjah1]

I found a work around. by using kubectl patch.

in the sts.tf

resource "kubernetes_stateful_set" "prometheus" {
  ...
          volume_mount {
            name             = "data"
            mount_path       = "/data"
            //sub_path_expr  = "$(POD_NAME)"
          }
  ...
  provisioner "local-exec" {
    command = "kubectl -n ${var.namespace} patch sts prometheus --patch \"$(cat ${path.module}/patch/subpathexpr.yaml)\""
  }
}

in the patch/subpathexpr.yaml

spec:
  template:
    spec:
      containers:
        - name: prometheus
          volumeMounts:
            - mountPath: "/data"
              subPathExpr: $(POD_NAME)

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