Add one-shot jobsets

There are jobsets that are evaluated only once, that is, after they've been evaluated, they're disabled automatically. This is primarily useful for doing releases: for instance, doing an evaluation with "officialRelease" set to "true" should be done only once.

Created by  Eelco Dolstra  on October 11, 2013
C3AG65SWAGDSML5MBD4ZPRCHI2LNNDN6IQZOI3HLYRJRPAW3QPAAC
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