Adding hiera default values

While Hiera is not set properly (on the first run, for example) applying **puppet::agent** class will fail with *"Error: Could not find data item puppet::agent::server in any Hiera data file and no default supplied at /tmp/vagrant-puppet/modules-0/puppet/manifests/agent.pp:36 on node nodename"*.

With this change the **puppet::agent** will be successfully applied, albeit using the default value.

I think 'puppet' is a good default server name to use.