Support for roles for MaxScale user

When assigning provileges to the user that is used for monitoring, to fetch authentication data and for replication it would be convenient to be able to use database roles. As it stands, this only works if the role in question is the default role for the user in question. I suggest that the settings for user/password for servers and sservices, monittoruser/monitorpw for servers and replication_user and replication_password for the mariadbmon monitor are complemented with a similar role parameter. When a role is specified, the role in question would be anabled after connection, i.e. by running SET ROLE <rolename>.