Add tags to deploy components by --tags option (#2960)

* Add tags for cert serial tasks

This will help facilitate tag-based deployment of specific components.

* fixup kubernetes node

docs/upgrades.md

@@ -81,3 +81,55 @@ kubernetes-apps/rotate_tokens role, only pods in kube-system are destroyed and
 recreated. All other invalidated service account tokens are cleaned up
 automatically, but other pods are not deleted out of an abundance of caution
 for impact to user deployed pods.
+
+### Component-based upgrades
+
+A deployer may want to upgrade specific components in order to minimize risk
+or save time. This strategy is not covered by CI as of this writing, so it is
+not guaranteed to work. 
+
+These commands are useful only for upgrading fully-deployed, healthy, existing
+hosts. This will definitely not work for undeployed or partially deployed
+hosts.
+
+Upgrade etcd:
+
+```
+ansible-playbook -b -i inventory/sample/hosts.ini cluster.yml --tags=etcd
+```
+
+Upgrade vault:
+
+```
+ansible-playbook -b -i inventory/sample/hosts.ini cluster.yml --tags=vault
+```
+
+Upgrade kubelet:
+
+```
+ansible-playbook -b -i inventory/sample/hosts.ini cluster.yml --tags=node --skip-tags=k8s-gen-certs,k8s-gen-tokens
+```
+
+Upgrade Kubernetes master components:
+
+```
+ansible-playbook -b -i inventory/sample/hosts.ini cluster.yml --tags=master
+```
+
+Upgrade network plugins:
+
+```
+ansible-playbook -b -i inventory/sample/hosts.ini cluster.yml --tags=network
+```
+
+Upgrade all add-ons:
+
+```
+ansible-playbook -b -i inventory/sample/hosts.ini cluster.yml --tags=apps
+```
+
+Upgrade just helm (assuming `helm_enabled` is true):
+
+```
+ansible-playbook -b -i inventory/sample/hosts.ini cluster.yml --tags=helm
+```