Originally, I had it in mind to move letsencrypt-auto inside this dir. However, now we'd like to copy it or link it to the root level, where people are used to finding it (at least for awhile). Since it would be confusing to have a letsencrypt-auto and a letsencrypt_auto right next to each other, we rename this folder.
They're now used only by the le-auto build process. The new public interface for OS-level bootstrapping is le-auto --os-packages-only, which dispatches by OS automatically. That obsoletes install-deps.sh as well, saving some repetition.
Also, switch to mustache-style templating to avoid colliding with shell variable references.
To optimize for the docker cache, we could later add a shim script that sources just deb_common.sh and calls its bootstrap function.
