Impement bulk polling of volumes

This implements Bulk volume polling using ideas presented by
justin in https://github.com/kubernetes/kubernetes/pull/39564

But it changes the implementation to use an interface
and doesn't affect other implementations.

pkg/volume/git_repo/git_repo.go

@@ -85,6 +85,10 @@ func (plugin *gitRepoPlugin) SupportsMountOption() bool {
 	return false
 }
 
+func (plugin *gitRepoPlugin) SupportsBulkVolumeVerification() bool {
+	return false
+}
+
 func (plugin *gitRepoPlugin) NewMounter(spec *volume.Spec, pod *v1.Pod, opts volume.VolumeOptions) (volume.Mounter, error) {
 	return &gitRepoVolumeMounter{
 		gitRepoVolume: &gitRepoVolume{