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/conversion.go

@@ -47,14 +47,6 @@ func init() {
 			out.Spec.DNSPolicy = in.DNSPolicy
 			out.Name = in.ID
 			out.UID = in.UUID
-			// TODO(dchen1107): Move this conversion to pkg/api/v1beta[123]/conversion.go
-			// along with fixing #1502
-			for i := range out.Spec.Containers {
-				ctr := &out.Spec.Containers[i]
-				if len(ctr.TerminationMessagePath) == 0 {
-					ctr.TerminationMessagePath = TerminationMessagePathDefault
-				}
-			}
 			return nil
 		},
 		func(in *BoundPod, out *ContainerManifest, s conversion.Scope) error {
@@ -65,12 +57,6 @@ func init() {
 			out.Version = "v1beta2"
 			out.ID = in.Name
 			out.UUID = in.UID
-			for i := range out.Containers {
-				ctr := &out.Containers[i]
-				if len(ctr.TerminationMessagePath) == 0 {
-					ctr.TerminationMessagePath = TerminationMessagePathDefault
-				}
-			}
 			return nil
 		},