Introduce SystemContainer to Kubelet ContainerManager.

This generalizes the handling of containers in the
ContainerManager.

Also introduces the ability to determine how much
resources are reserved for those system containers.

pkg/kubelet/runonce_test.go

@@ -89,6 +89,7 @@ func TestRunOnce(t *testing.T) {
 		os:                  kubecontainer.FakeOS{},
 		volumeManager:       newVolumeManager(),
 	}
+	kb.containerManager, _ = newContainerManager("", "", "")
 
 	kb.networkPlugin, _ = network.InitNetworkPlugin([]network.NetworkPlugin{}, "", network.NewFakeHost(nil))
 	if err := kb.setupDataDirs(); err != nil {