Testing: Improve confusing Jenkinsfile naming references in testing

[kmanning]

• There's the Jenkinsfile class, that is meant to be a reference to global setting (https://github.com/manheim/terraform-pipeline/blob/master/src/Jenkinsfile.groovy)
• The Jenkinsfile.init method accepts a reference to this, which is the actual underlying Jenkinsfile library. this is stored in a variable called original - not the best name. https://github.com/manheim/terraform-pipeline/blob/master/src/Jenkinsfile.groovy#L2
• Even more confusing, original needs to be mocked/stubbed/spied on during a lot of testing. To make this easier, a DummyJenkinsfile class is available to tests. https://github.com/manheim/terraform-pipeline/blob/master/test/DummyJenkinsfile.groovy

original is not a great name, and confounding it with DummyJenkinsfile and Jenkinsfile is even worse. Do something about this.

[kmanning]

Solution:

• this in Jenkinsfile.init(this) is a WorkflowScript
• Rename DummyJenkinsfile to MockWorkflowScript