Code review comment for lp:~trapnine/maas/b1459762

I get the feeling that access to a node is or should be more nuanced.

* A unowned node can only be updated by an admin, with one exception
  (IIRC): it can be acquired from the Ready state by a non-admin.

* An owned node can be modified by its owner. However, some parameters
  may only be modifiable by an owner who is also an admin.

This branch seems to prevent non-admins from modifying anything about a
node, and I'm not sure that's what we want.