[stable/datadog] Set DATADOG_HOST automatically

[PSanetra]

When setting up the Cluster Agent external metric server it is necessary to define DATADOG_HOST if you are using an EU account.

I think this environment variable should default to https://api.{{ .Values.datadog.site }}.

See https://docs.datadoghq.com/agent/cluster_agent/external_metrics/#custom-metrics-server and https://github.com/zorkian/go-datadog-api/blob/9e2c892691035a7685a16d5839910cfc54e42fb6/client.go#L55

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[stale]

This issue is being automatically closed due to inactivity.

[BitProcessor]

Still needed in version: 2.4.31 of the Helm chart:

    clusterAgent:
      env:
      - name: DATADOG_HOST
        value: "https://app.datadoghq.eu" 
      enabled: true
      metricsProvider:
        enabled: true

Not possible to set this parameter otherwise:
https://github.com/DataDog/helm-charts/blob/master/charts/datadog/templates/cluster-agent-deployment.yaml