Factor out API defaulting from validation logic

Currently, the validation logic validates fields in an object and supply default
values wherever applies. This change factors out defaulting to a set of
defaulting callback functions for decoding (see #1502 for more discussion).

 * This change is based on pull request 2587.

 * Most defaulting has been migrated to defaults.go where the defaulting
   functions are added.

 * validation_test.go and converter_test.go have been adapted to not testing the
   default values.

 * Fixed all tests with that create invalid objects with the absence of
   defaulting logic.

pkg/controller/replication_controller_test.go

@@ -239,7 +239,7 @@ func TestCreateReplica(t *testing.T) {
 	if err != nil {
 		t.Errorf("Unexpected error: %#v", err)
 	}
-	if !api.Semantic.DeepEqual(&expectedPod, actualPod) {
+	if !api.Semantic.DeepDerivative(&expectedPod, actualPod) {
 		t.Logf("Body: %s", fakeHandler.RequestBody)
 		t.Errorf("Unexpected mismatch.  Expected\n %#v,\n Got:\n %#v", &expectedPod, actualPod)
 	}
@@ -351,7 +351,7 @@ func TestWatchControllers(t *testing.T) {
 	var testControllerSpec api.ReplicationController
 	received := make(chan struct{})
 	manager.syncHandler = func(controllerSpec api.ReplicationController) error {
-		if !api.Semantic.DeepEqual(controllerSpec, testControllerSpec) {
+		if !api.Semantic.DeepDerivative(controllerSpec, testControllerSpec) {
 			t.Errorf("Expected %#v, but got %#v", testControllerSpec, controllerSpec)
 		}
 		close(received)