Make database validation optional

We don't want to validate the DB connection when building puppetdb nodes as we're building AMIs on AWS that won't necessarily have access to the RDS instance they will eventually use when instantiated.

This PR adds a couple of flags to make the db validation optional, one for database, one for read_database.

R.
You could avoid this entirely by setting the class's (`puppetdb::server::validate_db`) *before` param to `Ini_setting <| |>` ; collecting on all `Ini_setting`'s :)