Add hiera_config parameter

Another new parameter.

The default in Puppet is '$confdir/hiera.yaml', but for backwards compatibility I opted to make the default '' which prevents it from being added to puppet.conf.  If forcing the default to be visible (more transparent in my opinion) would not create an issue with backwards compatibility, then I can update PR to set default value in the module to match Puppet's default.
I would like to see the TODO gone before we merge it, depending on the solution we choose.