Add support for upgrading Postgres

Since cbee084ac1, this playbook supports Postgres 10.x,
but keeps existing Postgres-9.x installs on 9.x.

This playbook can now also be ran with `--tags=upgrade-postgres`
to make it upgrade from Postgres 9.x to 10.x (or other versions
in the future).

roles/matrix-server/tasks/main.yml

@@ -13,6 +13,10 @@
     - setup-main
     - setup-ssl
 
+- include: tasks/upgrade_postgres.yml
+  tags:
+    - upgrade-postgres
+
 - include: tasks/setup_postgres.yml
   tags:
     - setup-main