added new shared class LyricsApiBase and moved solr config there
used new class constant instead of global constant for artist type
used new class constant instead of global constant for album type
used new class constant instead of global constant for song type
removed LyricsApiConstants.php
query solr slaves for API methods
code review changes: made LyricsApiBase instance local variable
Merge pull request #3225 from Wikia/lyrics-api-lyrics-api-base-class

MOB-1288 Addressed notes from code review