Make instance api prettier.

@nacx @danbroudy lemme know if you agree this is prettier. If so, I will help do the other apis.
One hesitation here about moving '/zones/{zone}' up a level. [AggregatedList](https://cloud.google.com/compute/docs/reference/latest/instances/aggregatedList) is a very useful call in the InstanceApi that does not have a specific zone.