common.yaml

hosts:
   - database
   - some_other_host

common2.yaml

hosts:
   database:
     default_address: 192.0.2.1
   some_other_host:
     default_address: 192.0.2.2

site.pp

$hosts = lookup(hosts)
$default_ip = '192.0.2.42'
$hosts.each |String $host| {
  host{$host:
    address => $facts.fetch('override_database_ip', $default_ip)
  }
}

site2.pp

# expanded example
$hosts = lookup(hosts)
$hosts.each |String $host, Hash $config| {
  host{$host:
    address => $facts.fetch('override_database_ip', $config['default_address'])
  }
}