Allow timeout property to be provided per-build

What does this MR do?

This is a community contribution take over from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17534. It enabled users to specify timeout: per job in .gitlab-ci.yml file. This will override the project-level build timeout.

Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/14887

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Usage

run-sleep:
  script:
    - echo 'sleeping...'
    - sleep 60
    - echo 'test'
    - sleep 120
    - echo 'should not print this'
    - sleep 120
    - echo 'should not print this'
  timeout: 120