[concourse] in_parallel - support both array of steps; and explicit "steps" child

From https://concourse-ci.org/in-parallel-step.html

in_parallel: ([step] | config)

Two forms of configuration are supported. The simplest form is a simple array of other steps, like so:

in_parallel:
- get: a
- get: b

This is shorthand for the following configuration:

in_parallel:
  steps:
  - get: a
  - get: b

[mstockd]

Thank you @kdvolder, its working great in our case (using a format like:

- in_parallel:
      fail_fast: true
      steps:
      - task: task-1
        file: task-1.yml
        image: image-1

      - task: task-2
        file: task-2.yml
        image: image-2

[kdvolder]

If anyone wants to give the fix a try, you can download a snapshot .vsix file from this page:

http://dist.springsource.com/snapshot/STS4/nightly-distributions.html

Because of the somewhat strange typing… this was a bit more involved of a fix than the usual little ‘schema nitpicks’, so I really would appreciate someone to give it good try out. And let us know how it goes.