Allow to set up hiera test fixtures

Currently puppet reads hiera settings from a randomly created vardir or
from puppet config. For proper test fixtrues allow it to be set either
via RSpec.configuration or via :hiera_config in specific examples e.g.:

spec/fixtures/hiera/hiera.yaml:
  ---
  :backends:
    - yaml
  :hierarchy:
    - test

   :yaml:
      :datadir: 'spec/fixtures/hiera'

spec/fixtures/hiera/test.yaml:
  foovar: 'bar'

spec/spec_helper.rb:
  RSpec.configure do |c|
    ...
    c.hiera_config = File.join(fixture_path, 'hiera/hiera.yaml')
    ...
  end

This make testing modules with hiera lookups very simple.
Merge branch 'hiera_fixtures' of https://github.com/agx/rspec-puppet into agx-hiera_fixtures

Conflicts:
	lib/rspec-puppet.rb
	lib/rspec-puppet/example/class_example_group.rb
	lib/rspec-puppet/example/define_example_group.rb
	lib/rspec-puppet/example/host_example_group.rb
Check if a setting exists before setting it

hiera_config doesn't exist in Puppet < 3.x
Merge pull request #125 from rodjek/agx-hiera_fixtures

Agx hiera fixtures