Support parallel keyword for trigger jobs

Release notes

The Parallel matrix keywords allow you to run a job multiple times in parallel, using different variable values for each instance of the job, in this release we've added the Parallel keyword to a trigger job allowing you to run multiple downstream pipelines in parallel (child or multi projects pipelines) using different variables value for each downstream pipeline, giving speed and flexibility to your pipelines.

Problem to solve

I can't run parallel matrix in job that triggers child pipelines. This job fails in Linter:

test_pipeline:
  trigger:
    include: CI/tests.yml
  parallel:
    matrix:
      - HW_REVISION:
        - REV_A
        - REV_B
  variables:
    HW_REVISION: $HW_REVISION

With this error:

jobs:test_pipeline config contains unknown keys: parallel
jobs:test_pipeline config should contain either a trigger or a needs:pipeline

So looks like I can't use parallel keyword in job with trigger

Intended users

Anybody who use the matrix builds and want to move some jobs in separate pipeline.

User experience goal

Same user experience with matrix builds for any type of jobs

Proposal

I am not completely sure that this is not implemented already - maybe our gitlab server admin just should to configure some settings. But if it's not implemented - would be nice to have ability to launch multiple parallel child pipelines.

Further details

Permissions and Security

Documentation

Availability & Testing

Proposed solution

test_pipeline:
  trigger:
    include: CI/tests.yml
  parallel:
    matrix:
      - HW_REVISION:
        - REV_A
        - REV_B