rabbit/queue/tasks: allow specifying custom write_queues
    
    When creating a queue, the user is automatically created, but
    there is no control over what queues the user can write to. A
    user might need to write to queues other than ones prefixed with
    their name (e.g. some users need to write to 'ci.*' queues).
    This makes it possible to specify additional custom prefixes of
    queues the user will be permitted to write to.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>