os_capacity: Add tags to playbook, update vault docs

Previously the first deployment of a system with a Vault CA for internal
TLS and os_capacity enabled would fail when deploying HAProxy.
os_capacity deployment requires admin-openrc.sh to exist, but because of
the use of -kt haproxy the post-deploy tasks that create it will be
skipped.

This change fixes the issue by adding an os_capacity tag to the relevant
plays, and updating the Vault docs to skip the new tag when deploying
HAProxy.

Author: markgoddard

doc/source/configuration/vault.rst

@@ -111,7 +111,7 @@ Setup HAProxy config for Vault
 
    .. code-block::
 
-      kayobe overcloud service deploy -kt haproxy
+      kayobe overcloud service deploy --skip-tags os_capacity -kt haproxy
 
 Setup Vault HA on the overcloud hosts
 -------------------------------------

etc/kayobe/ansible/deploy-os-capacity-exporter.yml

@@ -1,6 +1,7 @@
 ---
 - name: Remove legacy os_exporter.cfg file
   hosts: network
+  tags: os_capacity
   gather_facts: false
   tasks:
     - name: Ensure legacy os_exporter.cfg config file is deleted
@@ -11,6 +12,7 @@
 
 - name: Deploy os-capacity exporter
   hosts: monitoring
+  tags: os_capacity
   gather_facts: false
   tasks:
     - name: Create os-capacity directory