Add DigitalOcean CSI driver

[xmudrii]

What this PR does / why we need it:

This PR:

• Adds the DigitalOcean CSI driver. The CSI driver is deployed automatically if .cloudProvider.external is enabled
• Adds the default StorageClass and VolumeSnapshotClass for the DigitalOcean CSI driver. The StorageClass and VolumeSnapshotClass can be deployed by enabling the default-storage-class embedded addon

The following manifest has been used for testing:

---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: csi-pvc-do
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 1Gi
  storageClassName: do-block-storage
---
apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
  - image: nginx
    imagePullPolicy: IfNotPresent
    name: nginx
    ports:
    - containerPort: 80
      protocol: TCP
    volumeMounts:
      - mountPath: /var/lib/www/html
        name: csi-data-do
  volumes:
  - name: csi-data-do
    persistentVolumeClaim:
      claimName: csi-pvc-do
      readOnly: false

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
xref #1710

Does this PR introduce a user-facing change?:

Add the DigitalOcean CSI driver. The CSI driver is deployed automatically if `.cloudProvider.external` is enabled
Add the default StorageClass and VolumeSnapshotClass for the DigitalOcean CSI driver. The StorageClass and VolumeSnapshotClass can be deployed by enabling the default-storage-class embedded addon

/assign @kron4eg

[kubermatic-bot]

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This pull-request has been approved by: xmudrii

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[kubermatic-bot]

LGTM label has been added.

Git tree hash: 61dcba7f2922c03f6cf82cb2a796b635b292cbe2

[kron4eg]

@xmudrii how do you think, should we update again? there is new release v4.0.0

[xmudrii]

@kron4eg I don't think it matters much, but I'll create a PR anyways.

[xmudrii]

Created #1820 to update the CSI driver.