Split postgres auth from per-minion telegraf creds

The old flow had two writers for each per-minion Telegraf password
(so-minion wrote the minion pillar; postgres.auth regenerated any
missing aggregate entries). They drifted on first-boot and there was
no trigger to create DB roles when a new minion joined.

Split responsibilities:

- pillar/postgres/auth.sls (manager-scoped) keeps only the so_postgres
  admin cred.
- pillar/telegraf/creds.sls (grid-wide) holds a {minion_id: {user,
  pass}} map, shadowed per-install by the local-pillar copy.
- salt/manager/tools/sbin/so-telegraf-cred is the single writer:
  flock, atomic YAML write, PyYAML safe_dump so passwords never
  round-trip through so-yaml.py's type coercion. Idempotent add, quiet
  remove.
- so-minion's add/remove hooks now shell out to so-telegraf-cred
  instead of editing pillar files directly.
- postgres.telegraf_users iterates the new pillar key and CREATE/ALTERs
  roles from it; telegraf.conf reads its own entry via grains.id.
- orch.deploy_newnode runs postgres.telegraf_users on the manager and
  refreshes the new minion's pillar before the new node highstates,
  so the DB role is in place the first time telegraf tries to connect.
- soup's post_to_3.1.0 backfills the creds pillar from accepted salt
  keys (idempotent) and runs postgres.telegraf_users once to reconcile
  the DB.

salt/orch/deploy_newnode.sls

@@ -25,8 +25,33 @@ manager_run_es_soc:
         - salt: {{NEWNODE}}_update_mine
 {% endif %}
 
+# so-minion has already added the new minion's entry to telegraf/creds.sls
+# via so-telegraf-cred before this orch fires. Reconcile the Postgres role
+# on the manager so the new minion can authenticate on its first highstate,
+# then refresh the minion's pillar so its telegraf.conf renders with the
+# freshly-written cred.
+manager_create_postgres_telegraf_role:
+  salt.state:
+    - tgt: {{ MANAGER }}
+    - sls:
+      - postgres.telegraf_users
+    - queue: True
+    - require:
+      - salt: {{NEWNODE}}_update_mine
+
+{{NEWNODE}}_refresh_pillar:
+  salt.function:
+    - name: saltutil.refresh_pillar
+    - tgt: {{ NEWNODE }}
+    - kwarg:
+        wait: True
+    - require:
+      - salt: manager_create_postgres_telegraf_role
+
 {{NEWNODE}}_run_highstate:
   salt.state:
     - tgt: {{ NEWNODE }}
     - highstate: True
     - queue: True
+    - require:
+      - salt: {{NEWNODE}}_refresh_pillar