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/deployment.yaml

@@ -75,4 +75,4 @@ spec:
             memory: 512Mi
           requests:
             cpu: 100m
-            memory: 64Mi
+            memory: 64Mi

cmd/flux/testdata/diff-kustomization/kustomization.yaml

@@ -8,4 +8,4 @@ secretGenerator:
  - literals:
    - username=admin
    - password=1f2d1e2e67df
-   name: secret-basic-auth
+   name: secret-basic-auth

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

cmd/flux/testdata/diff-kustomization/service.yaml

@@ -18,4 +18,4 @@ spec:
     - port: 9999
       targetPort: grpc
       protocol: TCP
-      name: grpc
+      name: grpc

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