postgresql: Configure for dedicated DB servers

I am going to use the *postgresql* group for the dedicated database
servers.  The configuration for those machines will be quite a bit
different than for the one existing machine that is a member of that
group already: the Nextcloud server.  Rather than undefine/override all
the group-level settings at the host level, I have removed the Nextcloud
server from the *postgresql* group, and updated the `nextcloud.yml`
playbook to apply the *postgresql-server* role itself.

Eventually, I want to move the Nextcloud database to the central
database servers.  At that point, I will remove the *postgresql-server*
role from the `nextcloud.yml` playbook.

postgresql.yml

@@ -1,3 +1,11 @@
 - hosts: postgresql
   roles:
+  - role: trustca
+    ca: dch-root-ca-r2
+  - role: postgresql-cert
+    tags:
+    - postgresql-cert
+  - role: wal-g-pg
+    tags:
+    - wal-g
   - postgresql-server