r/postfix: Support rewriting recipient addresses

The *postfix* role will now generate configuration and a lookup table
for [canonical address mapping][0] of email recipients.  To configure
the mapping, the `postfix_recipient_canonical_map` must be a dictionary
of source-target addresses, e.g.:

```yaml
postfix_recipient_canonical_map:
  my.bad.email@fake.test: my.real.email@example.com
```

[0]: https://www.postfix.org/ADDRESS_REWRITING_README.html#canonical

roles/postfix/handlers/main.yml

@@ -1,5 +1,7 @@
 - name: rehash postfix sasl passwd table
   command: postmap /etc/postfix/sasl_passwd
+- name: rehash postfix recipient canonical map
+  command: postmap /etc/postfix/recipient_canonical
 - name: restart postfix
   service:
     name=postfix