Update Ansible to call Docker GC

[mgrauer]

see https://github.com/girder/girder_worker/pull/97

At least provide the option to set up cron jobs at some scheduled interval to call Docker GC.

[zachmullen]

• https://github.com/DigitalSlideArchive/HistomicsTK/tree/master/ansible/roles/worker
• https://github.com/girder/covalic/tree/master/ansible/roles/celery

[zachmullen]

http://docs.ansible.com/ansible/cron_module.html