smtp-relay: PB to deploy Postfix SMTP relay

The `smtp-relay.yml` playbook configures Postfix on the managed node as
an SMTP relay.

group_vars/smtp-relay.yml

@@ -0,0 +1,8 @@
+smtp:
+  mode: relay
+  host: mail.pyrocufflink.net
+  port: 587
+  auth: plain
+  username: '{{ smtp_username }}'
+  password: '{{ smtp_password }}'
+  security: starttls

hosts

@@ -50,3 +50,5 @@ dns0.pyrocufflink.blue
 
 [zabbix:children]
 zabbix-server
+
+[smtp-relay]

smtp-relay.yml

@@ -0,0 +1,3 @@
+- hosts: smtp-relay
+  roles:
+  - postfix