Allow the search_api_solr service class to be overriden

By default Pantheon will only let you use the Pantheon Solr service with apachesolr and search_api_solr. You can't use an third party custom solr server, this is a problem for example if you are using both apachesolr and search_api_solr on the same site. This pull request changes the behavior of search_api_solr to be more like apachesolr, by using a variable for the service class instead of hardcoding it.