Use @config instead of @connection_params in FbAdapter

The initialize method of the adapter for [mysql](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb#L193), [pg](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb#L356), and [sqlite](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb#L130) sets a variable named `@config`. FbAdapter sets `@connection_params`.

This change will allow gems that expect the `@config` variable,  like [Octopus](https://github.com/tchandy/octopus/blob/master/lib/octopus/abstract_adapter.rb#L29) and [Multidb](https://github.com/atombender/multidb/blob/master/lib/multidb/configuration.rb#L42), to work.