Make node names consistent between the delegate and the procedural

When the procedural creates arnold nodes, it gives them the same name as in the usd file (unless a specific name is specified).
But in the render delegate, the node names are quite different, with a big prefix coming from the scene delegate index. This is going to cause problems when operators will be supported in Hydra, since operators rely on the node names to apply to specific nodes.

This ticket is about investigating what could be done to simplify the arnold node names in hydra, or what could be done to make both use cases consistent

Start by comparing the procedural's USD-file node naming with the render delegate's scene-delegate-index-prefixed naming, then review how Hydra operators identify nodes. Define the affected entry points and a consistent naming approach; done means both use cases produce names that operators can target.

Written by the indexing model from the issue text.