MultiJobBuilder.java: Sleep in the subtask polling loop

Without the sleep, the thread keeps polling and updating the subtask which causes a very high CPU usage.
This fixes [JENKINS-21649] and [JENKINS-21798]

Signed-off-by: Nicolas Morey-Chaisemartin <nmorey@kalray.eu>
Merge pull request #49 from nmorey/update-subbuild-poll

MultiJobBuilder.java: Sleep in the subtask polling loop