WIP: Initial implementation of step scripts

[imjasonh]

This adds a script field to the Step type which, when specified,
results in a temporary generated executable script file being generated
and invoked containing the specified script items.

The result is an easier-to-use scripting option for users who just want
to invoke simple scripts. Generated scripts should be an implpementation
detail and will not persist between steps.

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Fixes #781

Release Notes

Support "script mode" for steps, which allows users to specify simple scripts inline in their YAML definition.

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[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1alpha1/task_validation.go	77.1%	78.1%	1.0
pkg/reconciler/taskrun/resources/pod.go	89.7%	90.3%	0.5

[imjasonh]

This makes me wonder whether Script shouldn't just be a string, which can contain newlines.

That would change these to:

steps:
- image: ubuntu
  env:
  - name: FOO
    value: foooooooo
  script: |
    #!/bin/bash
    set -euxo pipefail
    echo "Hello from Bash!"
    echo FOO is ${FOO}
    for i in {1..10}; do
      echo line $i
    done

- image: node
  script: |
    #!/usr/bin/env node
    console.log("Hello from Node!")

- image: python
  script: |
    #!/usr/bin/env python3
    print("Hello from Python!")

- image: perl
  script: |
    #!/usr/bin/perl
    print "Hello from Perl!"

...which I think is a bit clearer. We'd only lose the ability to have YAML comments inline, but that's probably not worth it.

[imjasonh]

Closing in favor of #1432