Make it possible to forget generated jobs

By default, the job-dsl-plugin will delete all previously created jobs that are not generated by the current build execution. Unchecking the new box "Manage jobs" disable this behavior, leaving previously generated jobs as is.
Make it possible to forget generated jobs

Follow quidryan's comment, we should not disable the updating of templates.
Merge branch 'master' of https://github.com/jenkinsci/job-dsl-plugin
Make it possible to ignore existing jobs and choose action for deleted ones

Add all features discussed in pull request #26:
- possibility to ignore update to existing jobs
- when a deleted job is found, it can be either ignored, disabled (default) or deleted.
Merge pull request #26 from rborer/master

Make it possible to forget generated jobs