Configure per-runner timeout for shared-runners-manager-X on GitLab.com

As it's described in https://gitlab.com/gitlab-com/infrastructure/issues/4010#note_69372298, we should use our new per-runner job timeout feature and set the value for shared-runners-manager-X on GitLab.com.

Copying important part from the linked issue:

After a discussion we've decided that 3h timeout for Shared Runners would be a good compromise. It's 3 times more than most of currently active projects are using and still feels to be reasonable.

To apply this we should:

• choose the deadline,
• find which projects will be affected when we'll set the 3h timeout,
• notify owners of such projects, specifying when we'll update the timeout,
• find affected projects and notify owners before 2018-04-30: #4092
• apply the 3h timeout after 2018-05-08:
  • shared-runners-manager-1.gitlab.com
  • shared-runners-manager-2.gitlab.com
  • shared-runners-manager-3.gitlab.com
  • shared-runners-manager-4.gitlab.com