Optionally disable $datadir creation via new $manage_datadir switch.

This simple patch adds $manage_datadir (true/false) option to class hiera and enables to completely skip creation of $datadir. Reason is that me myself have datadir as a symlink and File[$datadir] { ensure => directory } unlinks $datadir and creates empty directory. Default is to manage $datadir.
s/unless/if/ or s/true/false/