When a v1 plugin was installed, several things were happening regarding tests:
1. v1 plugin loading was happening despite the TUTOR_IGNORE_ENTRYPOINT_PLUGINS
environment variable.
2. the CORE_READY event was not triggered because it was happening just once at
import time.
This was causing some tests to incorrectly load the MFE plugin.
The entrypoint in the "openedx" Docker image was used only to define the
DJANGO_SETTINGS_MODULE environment variable, based on SERVICE_VARIANT and
SETTINGS. We ditch SETTINGS in favour of defining explicitely
DJANGO_SETTINGS_MODULE.
The problem with the Docker entrypoint is that it was bypassed whenever we ran
`tutor local exec` or `tutor k8s exec`. By removing it we make it simpler for
end-users to run manage.py commands in kubernetes.