Promote kubelet graceful node shutdown to beta

- Change the feature gate from alpha to beta and enable it by default

- Update a few of the unit tests due to feature gate being enabled by
  default

- Small refactor in `nodeshutdown_manager` which adds `featureEnabled`
  function (which checks that feature gate and that
  `kubeletConfig.ShutdownGracePeriod > 0`).

- Use `featureEnabled()` to exit early from shutdown manager in the case
  that the feature is disabled

- Update kubelet config defaulting to be explicit that
  `ShutdownGracePeriod` and `ShutdownGracePeriodCriticalPods` default to
  zero and update the godoc comments.

- Update defaults and add featureGate tag in api config godoc.

With this feature now in beta and the feature gate enabled by default,
to enable graceful shutdown all that will be required is to configure
`ShutdownGracePeriod` and `ShutdownGracePeriodCriticalPods` in the
kubelet config. If not configured, they will be defaulted to zero, and
graceful shutdown will effectively be disabled.

pkg/features/kube_features.go

@@ -650,6 +650,7 @@ const (
 
 	// owner: @bobbypage
 	// alpha: v1.20
+	// beta:  v1.21
 	// Adds support for kubelet to detect node shutdown and gracefully terminate pods prior to the node being shutdown.
 	GracefulNodeShutdown featuregate.Feature = "GracefulNodeShutdown"
 
@@ -765,7 +766,7 @@ var defaultKubernetesFeatureGates = map[featuregate.Feature]featuregate.FeatureS
 	SizeMemoryBackedVolumes:                        {Default: false, PreRelease: featuregate.Alpha},
 	ExecProbeTimeout:                               {Default: true, PreRelease: featuregate.GA}, // lock to default in v1.21 and remove in v1.22
 	KubeletCredentialProviders:                     {Default: false, PreRelease: featuregate.Alpha},
-	GracefulNodeShutdown:                           {Default: false, PreRelease: featuregate.Alpha},
+	GracefulNodeShutdown:                           {Default: true, PreRelease: featuregate.Beta},
 	ServiceLBNodePortControl:                       {Default: false, PreRelease: featuregate.Alpha},
 	MixedProtocolLBService:                         {Default: false, PreRelease: featuregate.Alpha},
 	PreferNominatedNode:                            {Default: false, PreRelease: featuregate.Alpha},