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:
```yaml
  ---
  :backends:
    - yaml
  :hierarchy:
    - test

   :yaml:
      :datadir: 'spec/fixtures/hiera'
```

spec/fixtures/hiera/test.yaml:
```yaml
  foovar: 'bar'
```

spec/spec_helper.rb:
```ruby
  RSpec.configure do |c|
    ...
    c.hiera_config = File.join(fixture_path, 'hiera/hiera.yaml')
    ...
  end
```

This make testing modules with hiera lookups very simple.