MOB-1288 Addressed notes from code review

* addressed @aquilax note about not putting more globals into our codebase,
* addressed @idradm note about using solr slaves in API

We'll probably add more re-usable methods to `LyricsApiBase` once we start working on updating the index. There is also an open case: if use my custom "config getters" instead of ` \Wikia\Search\QueryService\Factory::getSolariumClientConfig()` method or not. I open for any suggestions ;)