[Buildbot] #3085: use some Jinja2 fanciness to make sure secrets exist

[Buildbot trac]

#3085: use some Jinja2 fanciness to make sure secrets exist
-----------------------------+-------------------------
Reporter:  dustin            |      Owner:
    Type:  enhancement       |     Status:  new
Priority:  patches-accepted  |  Milestone:  sys - other
 Version:  0.8.9             |   Keywords:  ansible
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 We're currently writing things like

 {{{
   - password: "{{ some_password }}"
 }}}

 with the assumption that `some_password` is in `secrets.yml`.  But for
 folks trying to reproduce externally, they have no good way to know to set
 that value -- and Jinja2 will helpfully substitute an empty string in this
 case.

 I bet we could use a Jinja2 filter or function to look up secrets.  This
 could avoid empty values and also make it easy to grep the codebase for
 required secrets.

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Ticket URL: <http://trac.buildbot.net/ticket/3085>
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