Make instance api prettier.

@nacx @danbroudy lemme know if you agree this is prettier. If so, I will help do the other apis.
cool call! Hmm so, there are a couple ways to address this.

1. `getInstanceApi(project, null).aggregatedList()` // add to InstanceApi and rejig to allow ignore zone
2. `listInstances()` // add to top-level GoogleComputeEngineApi interface

do either of these sound better, or something else?

If it is only one call that is aggregate, then seems 2 might work...