fix: externalOrgId failed unique check

[hemslo]

Field can't be optional and unique, multiple instance without this field populated lead to validation error

[
  {
    "filename": "/dependencies/ocm/crc/insights.yaml",
    "kind": "UNIQUE",
    "result": {
      "summary": "ERROR: /dependencies/ocm/crc/insights.yaml",
      "status": "ERROR",
      "reason": "DUPLICATE_UNIQUE_FIELD",
      "error": "The field 'externalOrgId' is repeated: ['/dependencies/ocm/crc/insights.yaml', '/dependencies/ocm/stage.yml', '/dependencies/ocm/production.yml']",
      "meta_schema_url": null,
      "ref": "/openshift/openshift-cluster-manager-1.yml",
      "schema_url": null,
      "ptr": "externalOrgId"
    }
  }
]

Fix #894

[jfreixa]

LGTM