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/api/serialization_test.go

@@ -158,6 +158,10 @@ func TestEncode_Ptr(t *testing.T) {
 		ObjectMeta: api.ObjectMeta{
 			Labels: map[string]string{"name": "foo"},
 		},
+		Spec: api.PodSpec{
+			RestartPolicy: api.RestartPolicy{Always: &api.RestartPolicyAlways{}},
+			DNSPolicy:     api.DNSClusterFirst,
+		},
 	}
 	obj := runtime.Object(pod)
 	data, err := latest.Codec.Encode(obj)
@@ -169,7 +173,7 @@ func TestEncode_Ptr(t *testing.T) {
 		t.Fatalf("Got wrong type")
 	}
 	if !api.Semantic.DeepEqual(obj2, pod) {
-		t.Errorf("Expected:\n %#v,\n Got:\n %#v", &pod, obj2)
+		t.Errorf("Expected:\n %#v,\n Got:\n %#v", pod, obj2)
 	}
 }