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/zz_generated.deepcopy.go

@@ -1409,7 +1409,9 @@ func (in *Container) DeepCopyInto(out *Container) {
 	if in.VolumeMounts != nil {
 		in, out := &in.VolumeMounts, &out.VolumeMounts
 		*out = make([]VolumeMount, len(*in))
-		copy(*out, *in)
+		for i := range *in {
+			(*in)[i].DeepCopyInto(&(*out)[i])
+		}
 	}
 	if in.LivenessProbe != nil {
 		in, out := &in.LivenessProbe, &out.LivenessProbe
@@ -5931,6 +5933,15 @@ func (in *Volume) DeepCopy() *Volume {
 // DeepCopyInto is an autogenerated deepcopy function, copying the receiver, writing into out. in must be non-nil.
 func (in *VolumeMount) DeepCopyInto(out *VolumeMount) {
 	*out = *in
+	if in.MountPropagation != nil {
+		in, out := &in.MountPropagation, &out.MountPropagation
+		if *in == nil {
+			*out = nil
+		} else {
+			*out = new(MountPropagationMode)
+			**out = **in
+		}
+	}
 	return
 }