Add tags

Add tags to allow more granular tasks filtering.
Add generator script for MD formatted tags found.
Add docs for tags how-to.

Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>

roles/etcd/tasks/main.yml

@@ -1,9 +1,13 @@
 ---
 - include: pre_upgrade.yml
+  tags: etcd-pre-upgrade
 - include: check_certs.yml
+  tags: [etcd-secrets, facts]
 - include: gen_certs.yml
+  tags: etcd-secrets
 - include: install.yml
   when: is_etcd_master
+  tags: upgrade
 - include: set_cluster_health.yml
   when: is_etcd_master
 - include: configure.yml