Don't treat IP address for a node special

There's no good reason why we should handle a node's IP address in any
special way; Razor's builtin logic certainly doesn't depend on it.

The big question: should we treat the other dedicated fields similarly ? (Yes, I am looking at you, hostname and root_password)

  * Instead of a dedicated node.ip_address field, store the node's IP address
    in node.metadata["ip"]
  * Remove the /svc/store endpoint that was used just for that
  * make the store_url helper a wrapper for store_metadata_url that is only
    used to update metadata entries (and not remove them)