(maint) Ensure HieraPuppet cached instance is reset

Running the following spec files in the exact given order

  - spec/unit/hiera_puppet_spec.rb
  - spec/unit/indirector/data_binding/hiera_spec.rb
  - spec/unit/parser/functions/lookup_spec.rb

could cause the lookup function spec tests to fail. The HieraPuppet
module would create an instance of Hiera and the specs would not
properly clean it up, the Puppet::DataBinding::Hiera class would
populate stored backend information with invalid data, and trying to
interact with hiera later on could cause bad cached data to be read,
causing failures that couldn't normally happen. This commit explicitly
wipes the HieraPuppet cached instance after the tests are run, in an
effort to prevent any other developers from spending thousands of hours
tracing through mazes of twisty spec passages.
Merge branch 'maint/master/reset-hiera_puppet-instance'