opensmtpd: configure root mail alias

And add a README.

roles/opensmtpd/README.md

@@ -0,0 +1,3 @@
+Install opensmtpd and configure it for sending local mail through a relay.
+
+The NetBox configuration context must define `mail_relay` with the relay hostname and `mail_root` with the email address where mail for root should be forwarded.

roles/opensmtpd/tasks/main.yml

@@ -27,6 +27,12 @@
     follow: true
   notify: restart smtpd
 
+- name: Configure root mail alias
+  lineinfile:
+    path: /etc/smtpd/aliases
+    regexp: "^#? *root:"
+    line: "root: {{ mail_root }}"
+
 - name: Enable mail server
   service:
     name: "{% if ansible_os_family == 'Alpine' %}smtpd{% else %}opensmtpd{% endif %}"