Add manual for adding node to existing PAI cluster

[Binyang2014]

To add node to existing PAI cluster we need to do:

1. run ansible-playbook -i ${HOME}/pai-deploy/cluster-cfg/hosts.yml docker-runtime-setup.yml --limit=nodelist to change docker daemon config
2. run ansible-playbook -i inventory/pai/hosts.yml cluster.yml --become --become-user=root -e "@inventory/gcrv100/openpai.yml" --limit=node list to add node to k8s cluster.
3. If you encounter cert related error. Please make sure all etcd master node has: /usr/local/bin/etcd-scripts and has /etc/ssl/etcd/openssl.conf. If not, copy it from other etcd masters
4. After add node to k8s cluster, if you want to remove CNI, please follow doc to remove it
5. Change PAI layout.yaml to add node to layout.yaml. Change services-configuration.yaml to update hived config
6. Update pai config by ./paictl config push -p /udpated-config
7. run ./paictl service start -n cluster-configuration to update config
8. Restart rest-server, hived, job-exporter

[suiguoxin]

Add-node issue for clusters with single master / etcd node is resolved in #5269.

This issue can be left here to track potential issues for clusters with multiple master/etcd nodes.