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/development.py

@@ -1,17 +1,20 @@
 from __future__ import absolute_import, unicode_literals
 
+from mayan.apps.smart_settings.utils import get_environment_setting
+
 from . import *  # NOQA
 
 ALLOWED_HOSTS = ['*']
 
 DEBUG = True
 
-CELERY_ALWAYS_EAGER = True
-CELERY_EAGER_PROPAGATES_EXCEPTIONS = CELERY_ALWAYS_EAGER
+CELERY_TASK_ALWAYS_EAGER = get_environment_setting(
+    name='CELERY_TASK_ALWAYS_EAGER', fallback_default='true'
+)
+CELERY_TASK_EAGER_PROPAGATES = CELERY_TASK_ALWAYS_EAGER
 
 EMAIL_BACKEND = 'django.core.mail.backends.console.EmailBackend'
 
-
 if 'rosetta' not in INSTALLED_APPS:
     try:
         import rosetta