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.