Implement support of the job:trigger keyword

[terszt]

The original issue: firecow/gitlab-ci-local#94

Goal: use job:trigger keyword in the pipeline definition as described in

• https://docs.gitlab.com/ee/ci/pipelines/downstream_pipelines.html
• https://docs.gitlab.com/ee/ci/yaml/index.html#trigger

Addition requirements:

• Print trigger identifier on each log output line
• We are not interested in supporting that gitlab-ci-local can trigger pipelines in other projects (only child pipeline is needed, multi-project pipeline not).

Steps:

• [ ] #2
• [ ] #3
• [ ] #4
• [ ] #5

[alexey-vostrikov]

Is where any progress?