Recent update in rspec-puppet removed some of the config interfaces for
old puppet versions[1]. This drops usage of these interfaces to resolve
the following error in unit tests.
```
An error occurred while loading ./spec/unit/provider/manila_spec.rb.
Failure/Error: c.manifest_dir = File.join(fixture_path, 'manifests')
NoMethodError:
undefined method `manifest_dir='...
```
[1] 316d95923c
Change-Id: If12107ec233a2214540c1fa2be6fedd920ec36de
2d8f3e3ef0