Fixing name of custom conf file mounted in DCA during migration

[CharlyF]

What does this PR do?

Upon converting v1alpha1 with a customConfig for the cluster agent, the file should be datadog-cluster.yaml so that it's properly processed by the binary.

currently we have:

      volumes:
      - configMap:
          defaultMode: 420
          items:
          - key: datadog-cluster.yaml
            path: datadog-cluster.yaml
          name: datadog-cluster-agent-custom-cm
        name: volume-custom-cluster-agent
...
        volumeMounts:
        - mountPath: /etc/datadog-agent/datadog.yaml
          name: volume-custom-cluster-agent
          readOnly: true
          subPath: datadog.yaml

with the wrong subPath.
leading to the /etc/datadog-agent/datadog.yaml being a directory since it's empty.

root@datadog-cluster-agent-56885cc567-bhmgm:/# cat /etc/datadog-agent/datadog.yaml
cat: /etc/datadog-agent/datadog.yaml/: Is a directory

Motivation

Fix edge case of migration.

Additional Notes

Anything else we should know when reviewing?

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