Add a listener to handle starting and completing child jobs

[sshelomentsev]

In some use cases, there is a need to retry the child job within this multi job. The current retry strategy that uses parsing of the console output isn't siutable.
Our use case:

1. We are want to integrate naginator plugin to rerun child job.
2. We are use our plugin, which keeps the job in progress until user fix the configuration in the case of a build failed. After the fix starts a new build.
   So, to do this we need to be able to start a new build in the current multijob build.

[jenkinsadmin]

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[emoshaya]

This looks great, does this allow you to manually restart a child job after the multi job build has completed? Would it then update the results of the multi job build?