Added 'Execute concurrent builds if necessary' option for jobs.

Wiki pages [Home](https://github.com/jenkinsci/job-dsl-plugin/wiki), [Job-DSL-Commands](https://github.com/jenkinsci/job-dsl-plugin/wiki/Job-DSL-Commands), and [Job-reference](https://github.com/jenkinsci/job-dsl-plugin/wiki/Job-reference) have been amended.