`--no-init-db` is used to silence the need for `--hostname` and
`--email` arguments; it is a proxy for "this is not a frontend host."
We would ideally like to use `has_class` to know if the user's
provided puppet classes are include an `app_frontend`, and thus
`--hostname` and `--email` are required -- but doing that requires
several other steps, and we would like this feedback to be immediate.
We make the presence of `--puppet-classes` equivalent to
`--no-init-db`, since nearly every configuration with
`--puppet-classes` does not install both a database and a frontend,
which is what is required to initialize a database.