Add ability to validate Hiera data & config

[sanfrancrisko]

Originally reported in PDK-742:

The Problem: users sometimes hiera YAML data or config files that are not valid.

We tell them they can use commit hooks in git to prevent these types of errors but often do not get adoption.

The Suggested Solution: PDK should be able to validate hiera data and config before it is committed into VCS

The Impact: we can prevent broken Hiera files from causing widespread compilation errors before it reaches the puppet master.