Allow a consistent defaulting mechanism for picking up the Jenkins under test and the plugins.

The sequence is:

- Follow JENKINS_WAR environment variable
- Try WORKSPACE/jenkins.war
- Try $(pwd)/jenkins.war

The plugins used will try for the first of:

- Follow PLUGINS_DIR
- A sibling directory of the resolved jenkins.war that is called `plugins`
- Try WORKSPACE/plugins
- Try $(pwd)/plugins