Add support for checking Hiera YAML files

This includes a new `hiera` Rake task that assumes a `hieradata` directory and
a `hiera.yaml` file in the root of the repo.

Rather than tackle thornier problems around validating Hiera config, such as -
 - using Hiera itself to validate (Hiera.new, Hiera::Config.load etc.)
 - whether we're on version >3 or 2.7 of Puppet
 - various Hiera failure modes (hiera_include, hiera_hash, etc.),

I've opted for a simple YAML parse test - for now! :)