feat: allow reading cluster_id from configMap

[samuelvl]

Phase1 onboarding creates the castai-agent-metadata configMap with the generated cluster id:

$ kubectl get cm -n castai-agent castai-agent-metadata -o yaml
apiVersion: v1
data:
  CLUSTER_ID: <my-cluster-id>
kind: ConfigMap
metadata:
  name: castai-agent-metadata
  namespace: castai-agent

We need to extend kvisor in order to use this same configMap to get cluster id.