Add ServerName = @fqdn to httpd.conf

On some nodes, we were seeing this warning: (thanks @gchaix)

```# service httpd restart
Stopping httpd:                                            [  OK  ]
Starting httpd: httpd: Could not reliably determine the server's fully qualified domain name, using 2001:4801:7817:72:f156:fa90:ff10:3a72 for ServerName [  OK  ]```

We fixed it by adding ServerName = fqdn in the httpd.conf.erb.