Add the possibility to set JOB_CLASS in get_scheduler call

[mato2000]

Currently the get_scheduler call supports only these arguments: name, queue, and interval. But when instances a new scheduler it takes the job_class from queue.job_class. If you want to change the job_class you should set the JOB_CLASS env variable globally, but you cannot do it by code. It would be better if get_scheduler have an extra job_class kwarg just like Scheduler class.

[selwin]

PR welcome!