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.

plugin/pkg/scheduler/factory/factory_test.go

@@ -195,7 +195,13 @@ func makeURL(suffix string) string {
 }
 
 func TestDefaultErrorFunc(t *testing.T) {
-	testPod := &api.Pod{ObjectMeta: api.ObjectMeta{Name: "foo", Namespace: "bar"}}
+	testPod := &api.Pod{
+		ObjectMeta: api.ObjectMeta{Name: "foo", Namespace: "bar"},
+		Spec: api.PodSpec{
+			RestartPolicy: api.RestartPolicy{Always: &api.RestartPolicyAlways{}},
+			DNSPolicy:     api.DNSClusterFirst,
+		},
+	}
 	handler := util.FakeHandler{
 		StatusCode:   200,
 		ResponseBody: runtime.EncodeOrDie(latest.Codec, testPod),