Parameter to not manage postgresql server

- add boolean parameter $manage_server to puppetdb::database::postgresql to conditionally not manage postgresql::server, allowing it to be managed separately, i.e. if we need to pass specific options to postgresql::server via an ENC, or if we want to manage the database but manage a (shared) postgresql server in one place
- remove the duplicate "manage_firewall" sections from README.md
- while we're here, fix the module description