Add --parallel option

Pass through the --parallel option to the QA framework.

This also removes the change to LOAD_PATH so that gitlab-qa can be used as a library. Without this change autoload failed to correctly load some files and so constants could not be resolved.

There is another MR to enable the --parallel option for all jobs: !255 (closed)

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29654 depends on this change. It's part of &1276