Set default puppet::server::servername to fqdn

Without servername set, the apache configuration file will point to an invalid filename, so this creates a working default setting. 

For example, without this you get errors like:

```shell
Error: /Stage[main]/Apache::Service/Service[httpd]/ensure: change from stopped to running failed: Could not start Service[httpd]: Execution of '/etc/init.d/apache2 start' retur
ned 1: Starting web server: apache2Syntax error on line 28 of /etc/apache2/sites-enabled/10-puppetmaster.conf:
SSLCertificateFile: file '/var/lib/puppet/ssl/certs/.pem' does not exist or is empty
```