Make instance api prettier.

@nacx @danbroudy lemme know if you agree this is prettier. If so, I will help do the other apis.
ok.. how about this. I'll revise the list javadoc when adding aggregate.
basically say, if you want to list across all zones go here?

p.s. I bet there's some nuance about aggregated list, like propagation
delay, right? Wondering if we should switch computeservice to use it or not.

p.p.s @nacx was working on a filter for images, and seems cleaner if there
were a cross-project aggregated image api. has that been requested?