gistfile1.pp

class nova (
  $database_connection = false,
) {
  # backward compat stuff
  # include ::nova::database
}

class nova::scheduler {
  include ::nova::database
}

class nova::database (
  $connection = undef,
) {

  $_connection = pick($connection, $::nova::database_connection, false)

  if $_connection {
    nova_config { 'database/connection': value => $_connection }
  } else {
    nova_config { 'database/connection': ensure => absent }
  }
}