The ACE_* settings from lms/envs/common.py are all ignored because they are
overloaded by the plugin settings. We were recently bitten by this, as we
discovered that the ACE_ROUTING_KEY was incorrectly set to 'edx.core.low'.
Here, we fix this default value and remove ACE_* settings from
lms/envs/common.py to avoid confusion.
See: https://github.com/overhangio/tutor/issues/439