Update openstack testing workflow

Openstack use berkshelf 3 and `berks install --path` don't working anymore, use `berks vendor` instead.
Strainer is no longer used and a Rakefile have been added to make the job, the default command for testing lint, style and unit is `bundle exec rake`.
Merge pull request #423 from PierreRambaud/patch-1

Update openstack testing workflow