Consider increasing limit of parallel keyword

The parallel keyword is currently limited to between 2 and 50. We should consider the ability to expand this (perhaps controlled by the instance or group administrator?) for use cases where it would need to be larger.

From internal discussion: https://gitlab.slack.com/archives/C0NFPSFA8/p1550260023160600

We currently max-out this limit on gitlab-ce builds:

/cc @ayufan @markpundsack