[Buildbot] #3120: Add capabilities to slaves

[Buildbot trac]

#3120: Add capabilities to slaves
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Reporter:  tardyp       |       Owner:
    Type:  enhancement  |      Status:  new
Priority:  minor        |   Milestone:  0.9.+
 Version:  master       |  Resolution:
Keywords:               |
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Comment (by tardyp):

 @sa2ajj: schedulers dont use slaves, no?

 @dustin. several people are doing such things privately in their conf,
 each of them reimplementing the same thing (at least metabb, and my team).
 capability is the solution every programer should come up with for those
 kind of problems. That's why it should be in core.

 Another usecase that is now complicated to implement is to address a
 subset of the slaves dynamically depending on a property.
 Multi-OS travis is one of them
 https://github.com/tardyp/buildbot_travis/blob/master/README.rst#todo
 Doing this now requires using a complicated nextSlave callback, while the
 requiredCapabilities callback would have been so straightforward,

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Ticket URL: <http://trac.buildbot.net/ticket/3120#comment:3>
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