Use knapsack to parallelize E2E tests run via package-and-qa
The QA E2E tests can be run in parallel processes on the same machine using that
--parallel option implemented in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29654 and gitlab-qa!250 (merged).
However, under those conditions some tests fail. Those failures are being investigated and fixed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30113
In the meantime, we can also parallelize the tests in
package-and-qa via knapsack, as we have for review apps in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28283.