In the context of edx-platform django settings,
devstack_docker is an alias to devstack. Both
are deprecated, technically, but we currently
want to remove all devstack-related settings files
except devstack.py (which tutor still uses). So,
in order to remove devstack_docker.py, we update
its references to devstack.py
Note: I had originally implemented this as a `warnings.warn()` call
directly in lms/envs/devstack.py and cms/envs/devstack.py, but for
whatever reason, those warnings were getting swallowed. System checks
display more prominently, anyway.
Part of: https://github.com/openedx/public-engineering/issues/247