Support installing a specific version of Certbot #34
Description
Blindly using git.latest seems inadvisable. Letsencrypt really made a big mistake in recommending this during its initial stages. Now Certbot uses proper semantic versioning, and we should support that as well. The latest version is tagged v0.9.3. At the very least, we should be able to specify the specific desired version in pillar, but ideally, we would be able to use e.g. "0.9.*" so that we pull in fixes as well, without risking automatically upgrading to any breaking changes.
The certbot-auto script will automatically fetch the latest version, but there is a --no-self-upgrade flag to prevent this. I'm not clear on whether this is the git master or a specific version tag.