Fix issue with glance multi store config
This updates the config to be compatible with kolla-ansible ussuri. The relevant release note:
Glance deployment now uses Multi-Store support. Users that have
default_stores in their service config overrides for glance-api.conf
should remove it and use default_backend if needed.
https://docs.openstack.org/releasenotes/kolla-ansible/ussuri.html
See: https://review.opendev.org/c/openstack/kolla-ansible/+/708114/