Initial commit to support Celery 4.3.0

Merges 55e9b2263c from versions/next
with code from GitLab issue #594 and GitLab merge request !55.

Thanks to Jakob Haufe (@sur5r) and Jesaja Everling (@jeverling)
for much of the research and code updates.

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/settings/testing/base.py

@@ -2,8 +2,8 @@ from __future__ import absolute_import, unicode_literals
 
 from .. import *  # NOQA
 
-CELERY_ALWAYS_EAGER = True
-CELERY_EAGER_PROPAGATES_EXCEPTIONS = True
+CELERY_TASK_ALWAYS_EAGER = True
+CELERY_TASK_EAGER_PROPAGATES = True
 BROKER_BACKEND = 'memory'
 
 COMMON_PRODUCTION_ERROR_LOG_PATH = '/tmp/mayan-errors.log'