title: Jenkins JobDSL multibranchPipelineJob change script path
body: <p>I am trying to create a multibranchPipelineJob in jobDSL, however the Jenkinsfile is at an alternative location to the default. 
  I have looked through the docs <a href="https://jenkinsci.github.io/job-dsl-plugin/#path/multibranchPipelineJob" rel="noreferrer">https://jenkinsci.github.io/job-dsl-plugin/#path/multibranchPipelineJob</a>
  and I cannot see a way to do this.
  Looking at the config.xml for a manually created multibranchPipelineJob the scriptPath is in the  section, but I cannot find a DSL method to set this.</p>
  
  <p>Can anyone offer any help on how to do this?
  Cheers</p>