Rename the plugin to "debian-cloud-chef"

As the main project has been renamed from [ec2debian-build-ami](https://github.com/andsens/ec2debian-build-ami) to [build-debian-cloud](https://github.com/andsens/build-debian-cloud) to reflect the fact it is not only tight to EC2, we also rename our plugin.
There is nothing EC2 specific, and the plugin should work with all providers.

This also updates all the references to the renamed build-debian-cloud.