Set fixture paths for unit tests
Set fixture paths for unit tests

This change defines manifest_dir and module_path expicitly in unit
tests so that modules installed under fixtures directory is properly
loaded.

Closes-Bug: #1930403
Change-Id: Idc7a8a5b663d1cccb54ff1f62e975582c81d9f82

openstack/puppet-gnocchi,openstack/puppet-gnocchi