LoadBalancer: support multiple masters

Add support for the following structure in DB config:

```
	'cluster-id' => array(
		'masters' => [
			'db-a1' => 1,
			'db-a2' => 1,
		],
		'slaves' => [
			'db-a3' => 1,
			'db-a4' => 1,
		]
	),
```

@Wikia/platform-team / @drozdo