bootstrapper: stop join-client earlier

[daniel-weisse]

Proposed change(s)

This PR fixes a rare issue where the join-client of the bootstrapper of the initial control-plane node of a new cluster would try to join on itself:

1. User runs constellation init
2. The init-server of a node receives the call and initiates the bootstrapping of Kubernetes
3. The join-client continues to run and keeps requesting the metadata API for available join-services
4. Kubernetes starts running, the init-server waits for all services to be fully installed and available
5. The metadata API reports the control-plane node as having a join-service available
6. The join-client of this control-plane tries to contact the joins-service (which runs on this node) to join the cluster (which it is already a part of)
7. The init-server returns and stops the join-client

This is fixed by instead of deferring the call to Clean() in the init-server, we call the function after we acquired the node lock (same procedure as in the join-client).

Additional Information

Log looks similar to the following

Feb 24 10:08:02 fedora bootstrapper[5610]: {"level":"INFO","ts":"2023-02-24T10:08:02Z","logger":"bootstrapper.initServer","caller":"initserver/initserver.go:117","msg":"Init called","peer":"35.191.16.151:58148"}
Feb 24 10:08:02 fedora bootstrapper[5610]: {"level":"INFO","ts":"2023-02-24T10:08:02Z","logger":"bootstrapper.initServer","caller":"initserver/initserver.go:199","msg":"Stopping"}
Feb 24 10:08:02 fedora bootstrapper[5610]: {"level":"INFO","ts":"2023-02-24T10:08:02Z","logger":"bootstrapper.join-client","caller":"joinclient/joinclient.go:177","msg":"Stopping"}
Feb 24 10:08:02 fedora bootstrapper[5610]: {"level":"INFO","ts":"2023-02-24T10:08:02Z","logger":"bootstrapper.join-client","caller":"joinclient/joinclient.go:161","msg":"Client stopped"}
Feb 24 10:08:02 fedora bootstrapper[5610]: {"level":"INFO","ts":"2023-02-24T10:08:02Z","logger":"bootstrapper.join-client","caller":"joinclient/joinclient.go:185","msg":"Stopped"}
Feb 24 10:08:15 fedora bootstrapper[5610]: {"level":"INFO","ts":"2023-02-24T10:08:15Z","logger":"bootstrapper.initServer","caller":"kubernetes/kubernetes.go:88","msg":"Installing Kubernetes components","version":"v1.25.6"}

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[daniel-weisse]

[3u13r]

LGTM