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/api/v1/zz_generated.conversion.go

@@ -5133,6 +5133,7 @@ func autoConvert_v1_VolumeMount_To_api_VolumeMount(in *v1.VolumeMount, out *api.
 	out.ReadOnly = in.ReadOnly
 	out.MountPath = in.MountPath
 	out.SubPath = in.SubPath
+	out.MountPropagation = (*api.MountPropagationMode)(unsafe.Pointer(in.MountPropagation))
 	return nil
 }
 
@@ -5146,6 +5147,7 @@ func autoConvert_api_VolumeMount_To_v1_VolumeMount(in *api.VolumeMount, out *v1.
 	out.ReadOnly = in.ReadOnly
 	out.MountPath = in.MountPath
 	out.SubPath = in.SubPath
+	out.MountPropagation = (*v1.MountPropagationMode)(unsafe.Pointer(in.MountPropagation))
 	return nil
 }