zabbix-server: Correctly set smtp_mynetworks

The `smtp_mynetworks` variable expects a list. Setting it to a string
resulted in each character in the string being interpreted as an item in
the list.

group_vars/zabbix-server.yml

@@ -5,4 +5,5 @@ pg_hba_extra:
   user: zabbix
   address: ''
   method: md5
-smtp_mynetworks: localhost
+smtp_mynetworks:
+- localhost