[Buildbot] #3155: document a way to "force" an ansible run

[Buildbot trac]

#3155: document a way to "force" an ansible run
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Reporter:  dustin  |      Owner:  sa2ajj
    Type:  task    |     Status:  new
Priority:  minor   |  Milestone:  sys - other
 Version:          |   Keywords:
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 (from `docs/ansible-todo.txt`)

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Ticket URL: <http://trac.buildbot.net/ticket/3155>
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