Add API for mount propagation.

In fact, this is one annotation + its parsing & validation. Appropriate
kubelet logic that uses this annotation is in following patches.

pkg/apis/settings/zz_generated.deepcopy.go

@@ -142,7 +142,9 @@ func (in *PodPresetSpec) DeepCopyInto(out *PodPresetSpec) {
 	if in.VolumeMounts != nil {
 		in, out := &in.VolumeMounts, &out.VolumeMounts
 		*out = make([]api.VolumeMount, len(*in))
-		copy(*out, *in)
+		for i := range *in {
+			(*in)[i].DeepCopyInto(&(*out)[i])
+		}
 	}
 	return
 }