Publish the list of Puppet modules used in Tails' infrastructure

Currently, the list of Puppet modules we use to configure our infrastructure is not public. Because of that, reproducing parts or the whole of our infra and contributing code is very difficult if one doesn't have access to the private repo.

One example where we want/need to reproduce part of the infra but (public) knowledge of the set of modules is needed can be seen here: puppet-tails!58 (comment 173724)

Some ways of doing this:

• Turn our modules/ directory itself into a public repository.
• Use r10k and maintain a public Puppetfile that describes the list of modules and checked-out refs.

Once this is done:

• Remove the Puppetfile added by puppet-tails!58 (d910d0fd)