Upgrade kitchen-inspec and improve parallel specs in gitlab_fluentd cookbook

In gitlab-cookbooks/gitlab_fluentd!226 (merged) we've limited the concurrency of the kitchen tests because they proved to be flaky. This could be circumvented by upgrading kitchen-inspec to a version larger than 2.2. We should try that out, and if it doesn't work out as expected, parallelize each suite in a separate ci job instead.

The following discussion from gitlab-cookbooks/gitlab_fluentd!226 (merged) should be addressed:

• @reprazent started a discussion:

  This increases the test suite duration quite a bit (~10 mins -> ~20 mins). But I'd prefer not updating kitchen-inspec here, I'd prefer to see if that fixes it, and if not parallelize this using 2 jobs instead.