Resolve "Rework database node selector to use externalLoads database names."

Closes #4919 (closed)

mysql> select db_cluster, db_server, db_name, db_user from wiki_sites where wiki_name = 'Cluster Wiki';
+------------+-----------+------------+---------+
| db_cluster | db_server | db_name    | db_user |
+------------+-----------+------------+---------+
| esports1   | NULL      | cluster_en | NULL    |
+------------+-----------+------------+---------+

$wgDBserver         = '127.0.0.1:3306';
$wgDBname           = 'cluster_en';
$wgDBuser           = 'bobdole';
$wgDBpassword       = 'bobdole';
$wgDBservers = [
[
'host' => $wgDBserver,
'dbname' => $wgDBname,
'user' => $wgDBuser,
'password' => $wgDBpassword,
'type' => 'mysql',
'driver' => 'aurora',
'flags' => DBO_DEFAULT,
'load' => 0
]
];
$wgLBFactoryConf['sectionsByDB']['cluster_en'] = 'esports1';

mysql> select db_cluster, db_server, db_name, db_user from wiki_sites where wiki_name = 'Applejack Wiki';
+------------+-----------+---------------------+---------+
| db_cluster | db_server | db_name             | db_user |
+------------+-----------+---------------------+---------+
| NULL       | 127.0.0.1 | applejack_gamepedia | bobdole |
+------------+-----------+---------------------+---------+
$wgDBserver         = '127.0.0.1:3306';
$wgDBname           = 'applejack_gamepedia';
$wgDBuser           = 'bobdole';
$wgDBpassword       = 'bobdole';
$wgDBservers = [
[
'host' => $wgDBserver,
'dbname' => $wgDBname,
'user' => $wgDBuser,
'password' => $wgDBpassword,
'type' => 'mysql',
'driver' => 'aurora',
'flags' => DBO_DEFAULT,
'load' => 0
]
];
$wgLBFactoryConf['sectionsByDB']['applejack_gamepedia'] = 'db1';