HNC: don't allow deletion of propagated objects

[adrianludwin]

In v0.5, the object controller was fixed to apply to all objects, but it only protected the "create" and "update" operations. Users can still delete objects, and while HNC will quickly recreate them, this does allow a very short gap during which the policies won't be applied.

I have a working patch already but I don't want to hold up v0.5.0 as we should probably test this for at least a week or so. We may fix this in v0.5.1.

/assign @adrianludwin