Allows a job to be executed when no other jobs are allowed to run

The pre-job steps are:

1. The plugin will initiate a shutdown mode which means that no other jobs can be started.
2. it waits until every job execept this one is finished and then starts to execute the job.

The post-job steps are:
1. Inactivates jenkins shutdown mode

Combining this plugin with for instance scheduled build will pause the build queue and let executing jobs
finish before invoking this job. When this job is done, it unpauses the build queue.


Plugin is created by Marco Ambu but almost completely rewritten by Sam Tavakoli.