Allow conditional statements (only, except, when) to run a job or not

Description

Currently jobs can be run/not with only/except based on branch specifiers. For many use cases this works like a charm. However sometimes it would be nice to run a job based on a condition. I.e. we only want to publish/promote artifacts for the nightly build

Proposal

Allow when, only/except to evaluate a condition based on variables

Links / references

Ansible supports something similar with their when statement

Documentation blurb

Overview

What is it?

Expanding conditional execution of jobs

Why should someone use this feature?

Some jobs need to run conditional, but there aren't always specific branches for this condition

What is the underlying (business) problem?

We publish artifacts during the nightly build. This runs from master, but not all master builds should publish artifacts.

How do you use this feature?

...
publish:
  stage: publish
  script:
    - ./publish.sh
  when: $PUBLISH_ARTIFACTS==true
...

In this case it can be triggered like so:

curl -X POST -F token=${ACCESS_TOKEN} \
  -F ref=master \
  -F variables[PUBLISH_ARTIFACTS]=true \
   https://gitlab.example.com/api/v4/projects/${project_id}/trigger/pipeline

Use cases

Who is this for? Provide one or more use cases.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.