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/watch/json/encoder_test.go

@@ -19,7 +19,6 @@ package json
 import (
 	"bytes"
 	"io/ioutil"
-	"reflect"
 	"testing"
 
 	"github.com/GoogleCloudPlatform/kubernetes/pkg/api"
@@ -66,7 +65,7 @@ func TestEncodeDecodeRoundTrip(t *testing.T) {
 			t.Errorf("%d: unexpected error: %v", i, err)
 			continue
 		}
-		if !reflect.DeepEqual(testCase.Object, obj) {
+		if !api.Semantic.DeepDerivative(testCase.Object, obj) {
 			t.Errorf("%d: expected %#v, got %#v", i, testCase.Object, obj)
 		}
 		if event != testCase.Type {