[Buildbot] #3374: Whitelist config keys exposed in the config endpoint

[Buildbot trac]

#3374: Whitelist config keys exposed in the config endpoint
---------------------+-------------------
Reporter:  dustin    |      Owner:
    Type:  defect    |     Status:  new
Priority:  critical  |  Milestone:  0.9.0
 Version:  0.9.0b4   |   Keywords:
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 `master/buildbot/www/config.py` currently exposes all of `c['www']` with a
 few exceptions.  It'd be nice, instead, to only expose what must be
 exposed, so that any secrets added to other keys don't end up on the
 Internet.

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Ticket URL: <http://trac.buildbot.net/ticket/3374>
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Buildbot: build/test automation