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

[Buildbot trac]

#3085: use some Jinja2 fanciness to make sure secrets exist
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Reporter:  dustin            |       Owner:
    Type:  enhancement       |      Status:  new
Priority:  patches-accepted  |   Milestone:  sys - other
 Version:  0.8.9             |  Resolution:
Keywords:  ansible           |
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Comment (by skelly):

 The default in ansible is to fail if the variable is undefined. This can
 be disabled but there is a
 [https://docs.ansible.com/playbooks_filters.html#forcing-variables-to-be-
 defined filter] available, `mandatory`, that makes the variable required
 to be set.

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Ticket URL: <http://trac.buildbot.net/ticket/3085#comment:2>
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