Move GCE to v1beta16 API

GCE's got a new API version, v1beta16 - https://developers.google.com/compute/docs/transition-v1beta16 - there's not a huge difference. The only real changes are in the load balancer, which we're not implementing yet, and moving around quotas from zones to regions - and we didn't actually have quotas in zones in the first place! Nonetheless, it'd be good to bump the API version, since they tend to vanish old ones.

For 1.6.3, this'll just mean s/v1beta15/v1beta16/g. For 1.7.0, I'm also adding the region-level quotas.