Enable SSH pipelining.

NOTE: This improves speed, but requires that none of the playbooks use a TTY.
Make deploy user configurable. Delete other admin users/SSH keys.
Fix hardcoded `deploy` user
Bootstrap server and immediately hand off to {{deploy}} user.

This has made `build-server.yml` much cleaner, since we don't need to hop to and from sudo.
We'll have to mark each task (e.g. apt) as requiring sudo.