Add parallel version of custom job

We currently can't retry a job with custom variables (e.g., this job: https://gitlab.com/gitlab-org/gitlab-qa/merge_requests/362)

See https://gitlab.com/gitlab-org/gitlab/issues/32712 and https://gitlab.com/gitlab-org/gitlab/issues/31367

We need that ability to check for flakiness.

For now, let's have the option to run a custom job 10 times in parallel.

The downside to this option is that you have to specify the custom variables for each of the 10 custom jobs (or however many of the 10 jobs you want to run)