Move RHEL (un)registration to NodeExtraConfig

Move RHEL (un)registration to NodeExtraConfig

Currently, we have a problem because the unregistration happens in the
"post deploy" phase, which works fine when the top-level stack is being
deleted, but not when the ResourceGroup of servers is being scaled down,
because then the normal "post deploy" update ordering is respected and
we try to unregister after the corresponding server has been deleted.

So, instead, register/unregister each node inside the unit of scale,
e.g the role template being scaled down, which is possible via the new
NodesExtraConfig interface, which means unregistration will take
place at the right time both on stack delete and on scale-down.

Change-Id: I8f117a49fd128f268659525dd03ad46ba3daa1bc