Rename script + add support for Google Compute Engine

_(The Debian wiki's existing documentation will probably need to be updated together with this merge, to replace former references to the script name `ec2debian-build-ami` with `build-debian-cloud ec2`.)_

This commit is the result of collaboration among David McWherter, Anders
Ingemann, and me. The rename to build-debian-cloud is at Anders's
initiative.

David's and my work on this is Copyright 2013 Google, Inc. and is
released under the Apache License 2.0.