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/secret.yaml

@@ -9,4 +9,4 @@ metadata:
       kustomize.toolkit.fluxcd.io/namespace: {{ .fluxns }}
   name: db-user-pass-bkbd782d2c
   namespace: default
-type: Opaque
+type: Opaque