Add broker name in Schedule and enhanced Queued Tasks list display admin

[telmobarros]

This PR is somehow related to #107 . When we have several queues, the scheduled task is launched by a random queue (what is fine) but the task is also launched to be executed by the same queue what sometimes can delay it if the queue is very busy:

django_q/cluster.py (here)

# Call scheduler once a minute (or so)
  counter += cycle
  if counter >= 30 and Conf.SCHEDULER:
      counter = 0
      scheduler(broker=self.broker)

(and here)

def scheduler(broker: Broker = None):
    """
    Creates a task from a schedule at the scheduled time and schedules next run
    """
    if not broker:
        broker = get_broker()

.....

q_options["broker"] = broker

My PR solves it by assigning a variable 'broker_name' to 'q_options' dict when defining the Schedule Object. If the broker_name refers to an invalid/not running queue it fallbacks to the existing code.

Regardind the enhancement in Queued Tasks list display admin it is simply a filter by key.

[codecov-io]

Codecov Report

Merging #502 (1d6d996) into master (6f0b24c) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #502      +/-   ##
==========================================
+ Coverage   90.52%   90.54%   +0.01%     
==========================================
  Files          47       47              
  Lines        3082     3088       +6     
==========================================
+ Hits         2790     2796       +6     
  Misses        292      292              

Impacted Files	Coverage Δ
django_q/admin.py	98.18% <100.00%> (+0.03%)	⬆️
django_q/cluster.py	91.73% <100.00%> (+0.09%)	⬆️

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[Koed00]

I like it. Would like to see a little snippet in the docs for this to make it easier for other people with the same issue to use.

[telmobarros]

Done.

Thank you for your contribution with this amazing tool!

[Koed00]

Thanks you for your contribution! Without people like you helping out, I couldn't maintain this.