Spotinst: Use BDM to configure the root volume size at VNG level

[liranp]

This PR removes the setting of the obsolete rootVolumeSize field in order to resolve a validation error returned from the Spot backend:

{
  "request": {
    "id": "dedd14d9-7f56-4bee-951b-redacted",
    "url": "/ocean/aws/k8s/launchSpec",
    "method": "POST",
    "timestamp": "2021-04-06T19:33:53.885Z"
  },
  "response": {
    "status": {
      "code": 400,
      "message": "Bad Request"
    },
    "errors": [
      {
        "code": "ValidationError",
        "message": "\"rootVolumeSize\" must not exist simultaneously with [blockDeviceMappings]",
        "field": "body:launchSpec"
      }
    ]
  }
}

[hakman]

/approve

[hakman]

/approve

[k8s-ci-robot]

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