Allow admin user to explicitly unschedule the node

Setting Unschedulable on the node will not touch any existing pods
on the node but will block scheduling of new pods on the node.

pkg/api/validation/validation_test.go

@@ -2106,6 +2106,21 @@ func TestValidateMinionUpdate(t *testing.T) {
 				Labels: map[string]string{"Foo": "baz"},
 			},
 		}, true},
+		{api.Node{
+			ObjectMeta: api.ObjectMeta{
+				Name: "foo",
+			},
+			Spec: api.NodeSpec{
+				Unschedulable: false,
+			},
+		}, api.Node{
+			ObjectMeta: api.ObjectMeta{
+				Name: "foo",
+			},
+			Spec: api.NodeSpec{
+				Unschedulable: true,
+			},
+		}, true},
 	}
 	for i, test := range tests {
 		errs := ValidateMinionUpdate(&test.oldMinion, &test.minion)