Add the ability to change the Celery task queue

[vtoupet]

First, let me thank you for this great django app I've just discovered. This is brilliant. In my project, I am using Celery a lot. I have a lot of celery queues. That would be great to have the ability to change the task queue that will be used (when it is not defined, the 'celery' named queue will be used).

You might want to have a setting similar as RASTER_USE_CELERY (for example RASTER_CELERY_QUEUE).

What do you think?

Would you like me to do a PR?

[yellowcap]

Thanks @vtoupet - always glad to hear that our package is useful! By all means - pull requests are always welcome. Your request makes sense, I have not come across the need of multiple queues yet but I can see how it can be useful in larger apps.