In Kolla CI K-A upgrade job needs docker_custom_config override
as docker_registry var is being used both for docker daemon
config - for kolla images build, and kolla-ansible container images
sources - where we're using quay.io mirror.
docker_custom_config gets precedence in docker daemon
configuration.

docker_custom_config was removed in [1].

[1] https://review.opendev.org/c/openstack/kolla-ansible/+/904067

Change-Id: I1e890223faf25b1169a49e22a9529f90806d2f3a