Adding support for hiera data lookup

This patch adds option to use hiera in order to make this module
OS independent using data-in-modules technique.

Signed-off-by: Lukas Bednar <lbednar@redhat.com>
add 1) in case I will add trailing space to 'path_to_exports_d' variable, should I remove the slashes in path composition ? like:

file { "${::nfs::params::path_to_exports_d}README":