[Buildbot] #3022: Provide a MatrixBuilderFactory

[Buildbot trac]

#3022: Provide a MatrixBuilderFactory
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Reporter:  Ben          |       Owner:
    Type:  enhancement  |      Status:  new
Priority:  minor        |   Milestone:  0.9.+
 Version:  master       |  Resolution:
Keywords:               |
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Comment (by dustin):

 Let's leave it external, or in contrib/, and see what people think -- it's
 easy enough to incorporate into a config that way.  If it becomes popular,
 we can move it into core.

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Ticket URL: <http://trac.buildbot.net/ticket/3022#comment:5>
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