v6.0.15

Changelog:

cb5a675 cephadm-adopt: use cephadm_ssh_user for ssh user
969e41f cephadm-adopt: set cephadm registry login info
432ab37 purge: add remove_docker tag
0897c08 purge: add container_binary needed for zap osds
121bb58 ceph-defaults: set quay.io as the default registry
ac6604a purge-dashboard: remove cid files
1626caa ceph-container-engine: allow override container_package_name and container_service_name