containerd docker hub registry mirror support (#6962)

* containerd docker hub registry mirror support

* add docs

* fix typo

* fix yamllint

* fix indent in sample
and ansible-playbook param in testcases_run

* fix md

* mv common vars to tests/common/_docker_hub_registry_mirror.yml

* checkout vars to upgrade tests

roles/container-engine/containerd/templates/config.toml.j2

@@ -62,7 +62,7 @@ disabled_plugins = ["restart"]
 [plugins.cri.registry.mirrors]
 {% for registry, addr in containerd_config.registries.items() %}
 [plugins.cri.registry.mirrors."{{ registry }}"]
-  endpoint = ["{{ addr }}"]
+  endpoint = ["{{ ([ addr ] | flatten ) | join('","') }}"]
 {% endfor %}
 {% endif %}