Use overrides methods from KubeSpawner

[ryanlovett]

Summary

We apply overrides in our CustomKubeSpawner to change things like memory allocations and mount paths. The upstream KubeSpawner also has methods for doing such things. We should call upstream methods and stop using our own code. Hopefully we can get to a point where we don't need a customer spawner.

I believe the appropriate exposed method is now load_user_options. We may need to override this in our subclass since we do not use forms to set configuration.

User Stories

• Where appropriate, we should either upstream our code or use upstream's code. This lessens development and maintenance time.

Acceptance criteria

• When we can remove our apply_override method and have the same functionality.

[shaneknapp]

PR to move the spawn delay and timer/patience message upstream: jupyterhub/kubespawner#825