[google] global IP addresses

I should've really added this as part of #390 but didn't catch it until after it had been merged.

Google Compute Engine typically has region-based IP addresses, but with the introduction of HTTP load-balancing, a `global` static IP addresses have been introduced.  This PR is a small change to allow the `GCEAddress` class to allow for 'global' to be specified as the region.

New tests added to ensure both the original 'region-only' and new 'global' addresses are supported.