Update openstack-cinder-csi helm chart for multi cloud support

[guillaumebernard84]

What this PR does / why we need it:

Hello, this PR modifies the openstack-cinder-csi helm chart to allow to use it with the multi cloud feature: #2551

It fixes two problems that were preventing to use the chart with multi cloud:

1. When runing the cinder-csi-plugin with multi-region, you need to specify different extraArgs on the cinder-csi-plugin containers of the deployment and the daemonset. It wasn't possible with the current version. Now you can, like this:

nodePlugin:
  extraArgs: |-
    - --cloud-name=region-one
    - --additional-topology
    - topology.kubernetes.io/region=region-one
controllerPlugin:
  extraArgs: |-
    - --cloud-name=region-one
    - --cloud-name=region-two

You still need to create the second daemonset manually, but at least you can use the chart. If this PR is accepted, I'll work on a new one to allow the creation of the second daemonset.

2. If you use the resizer and the snapshotter, you will need them to be able to read the secrets you defined in the storage class' annotations in order to determine which cloud to address. This was hardcoded in controllerplugin-rbac.yaml, now you can specify it like this:

snapshotter:
  extraRbac:
    - apiGroups: [""]
      resources: ["secrets"]
      verbs: ["get", "list"]
resizer:
  extraRbac:
    - apiGroups: [""]
      resources: ["secrets"]
      verbs: ["get", "list", "watch"]

I also moved the comments from controllerplugin-rbac.yaml to the values file.

I added these info to the multi-region-clouds.md documentation file.

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Hi @guillaumebernard84. Thanks for your PR.

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