Celery commom config

[mkiwala]

Closes genome/ptero-workflow#212

I tested this PR locally against the ptero-petri repo. I checked that configuration is being set correctly by adding a debug print to the petri celery_app.py which prints the results of app.conf.humanize(with_defaults=True, censored=False). Here is an abridged version of that debug print showing the configuration variables that we're setting from ptero-common now, and the new broker heartbeat configuration:

11:10:28 http_worker.1       | CELERY_TASK_SERIALIZER: 'json'
11:10:28 http_worker.1       | CELERY_RESULT_SERIALIZER: 'json'
11:10:28 http_worker.1       | BROKER_TRANSPORT_OPTIONS: {
11:10:28 http_worker.1       |     'confirm_publish': True}
11:10:28 http_worker.1       | BROKER_HEARTBEAT: 300.0
11:10:28 http_worker.1       | CELERY_EVENT_SERIALIZER: 'json'
11:10:28 http_worker.1       | CELERY_ACCEPT_CONTENT: ['json']
11:10:28 http_worker.1       | CELERY_ACKS_LATE: True
11:10:28 http_worker.1       | BROKER_HEARTBEAT_CHECKRATE: 5.0

[mkiwala]

Merging immediately so that I can push out changes for ptero-lsf today.