Add graceful shutdown when interrupted

If implemented this permit restoring a clean state in case of signal
interruption.

Signed-off-by: Soule BA <soule@weave.works>

cmd/flux/testdata/diff-kustomization/value-sops-secret.yaml

@@ -8,4 +8,4 @@ metadata:
       kustomize.toolkit.fluxcd.io/namespace: {{ .fluxns }}
   name: podinfo-token-77t89m9b67
   namespace: default
-type: Opaque
+type: Opaque