Improve Celery log output

[dvanaken]

Changes:

• Removed extremely verbose log messages in celery tasks (specifically, the ones that printed out entire matrices of knob/metric data)
• Set celery.beat log level to INFO so we don't see the frequent heartbeat messages
• Added 'console' handler to celery logging config
• Updated the way the startcelery management command handles celery output when the list of logging handlers includes 'console'
• Reworded warnings in the stopcelery management command to make sure it's clear they are warnings and not errors