zabbix-server: Allow SMTP relay from any loopback

The Zabbix server resolves *localhost* to `::1`, but Postfix resolves it
to `127.0.0.1`.  This causes Postfix to reject incoming mail from Zabbix
with "Relay access denied."  Explicitly setting the `mynetworks` setting
to include both the IPv4 and IPv6 loopback addresses will ensure that no
mail is rejected from local processes, regardless of how name resolution
happens.

group_vars/zabbix-server.yml

@@ -6,4 +6,5 @@ pg_hba_extra:
   address: ''
   method: md5
 smtp_mynetworks:
-- localhost
+- '[::1]'
+- 127.0.0.7