Rename /SSH/authorized_key to better match VirtualBox convention.
Add ec2-run-user-data from build-debian-cloud.

https://github.com/andsens/build-debian-cloud/blob/9b2dd24eaa922cb0e4cd02416e8ab32339d508e9/init.d/ec2-run-user-data
Adapt ec2-run-user-data for VirtualBox.