Downloading region from /latest/meta-data/placement/availability-zone

The current code used us-east-1 as the region but there are many others,
so now we're downloading the region from amazon's API.

Sometimes the answer ends with a number different from the regions
available at: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-regions-availability-zones.html
so we're going to trim the last element.