fix: replica set error from pymongo 3.10 -> 3.12 upgrade

The pymongo dependency for edx-platform was updated (3.10.1 to 3.12.3)
in openedx/edx-platform#30569

This caused the following error when running the edx-platform database
migration split_modulestore_django.0002_data_migration  as part of
`tutor dev quickstart`:

  pymongo.errors.ServerSelectionTimeoutError: client is configured to
  connect to a replica set named '' but this node belongs to a set named
  'None', Timeout: 30s, Topology Description: <TopologyDescription id:
  62bdbaf182687350acf1aeec, topology_type: Single, servers:
  [<ServerDescription ('mongodb', 27017) server_type: Unknown, rtt:
   None, error=ConfigurationError("client is configured to connect to a
   replica set named '' but this node belongs to a set named 'None'")>]>

This commit explicitly sets replicaSet to None to indicate that it's a
standalone MongoDB instance. I also had to remove the CONTENTSTORE entry
from auth.yml because edx-platform's devstack.py assumes it has a
non-null value (set in common.py), and devstack.py executes before
tutor's development.py can set this replicaSet value.

tutor/templates/apps/openedx/config/partials/auth.yml

@@ -1,8 +1,7 @@
 SECRET_KEY: "{{ OPENEDX_SECRET_KEY }}"
 AWS_ACCESS_KEY_ID: "{{ OPENEDX_AWS_ACCESS_KEY }}"
 AWS_SECRET_ACCESS_KEY: "{{ OPENEDX_AWS_SECRET_ACCESS_KEY }}"
-CONTENTSTORE: null
-DOC_STORE_CONFIG: null
+DOC_STORE_CONFIG: {}
 {{ patch("openedx-auth") }}
 XQUEUE_INTERFACE:
   django_auth: null

tutor/templates/apps/openedx/settings/partials/common_all.py

@@ -16,6 +16,7 @@
     "password": None,
     {% endif %}
     "db": "{{ MONGODB_DATABASE }}",
+    "replicaSet": None,
 }
 DOC_STORE_CONFIG = mongodb_parameters
 CONTENTSTORE = {