MaxScale doesn't understand roles

When a user is assigned a role which grants access to a specific database, MaxScale will refuse to grant access for that user.

Steps to reproduce

1. Execute following SQL on master

   CREATE DATABASE my_db;

   CREATE ROLE dba;

   GRANT SELECT ON my_db.* TO dba;

   CREATE USER 'test'@'%' IDENTIFIED BY 'test';

   SET DEFAULT ROLE dba FOR 'test'@'%';

2. Start MaxScale with any configuration that uses the master where the SQL was executed
3. Connect to MaxScale as 'test'@'%' with the default database my_db

   mysql -u test -ptest -h $maxscale_host -P $maxscale_port -D my_db