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

docs/containerd.md

@@ -0,0 +1,31 @@
+# conrainerd
+
+[containerd] An industry-standard container runtime with an emphasis on simplicity, robustness and portability
+Kubespray supports basic functionality for using containerd as the default container runtime in a cluster.
+
+_To use the containerd container runtime set the following variables:_
+
+## k8s-cluster.yml
+
+```yaml
+container_manager: containerd
+```
+
+## Containerd config
+
+Example: define registry mirror for docker hub
+
+```yaml
+containerd_config:
+  grpc:
+    max_recv_message_size: 16777216
+    max_send_message_size: 16777216
+  debug:
+    level: ""
+  registries:
+    "docker.io":
+      - "https://mirror.gcr.io"
+      - "https://registry-1.docker.io"
+```
+
+[containerd]: https://containerd.io/