[Buildbot] #2875: support retryOnFailure

[Buildbot trac]

#2875: support retryOnFailure
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Reporter:  dustin       |       Owner:
    Type:  enhancement  |      Status:  new
Priority:  major        |   Milestone:  0.9.0
 Version:  0.8.9        |  Resolution:
Keywords:               |
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Comment (by stibbons):

 How many retries to do? One may not be enough. And yes, what is the result
 status in case of failure after retries

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Ticket URL: <http://trac.buildbot.net/ticket/2875#comment:3>
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