Part of the file name template is eaten up by Python's `%` formatting,
resulting in log files missing the `s` of "stderr" and "stdout".

supervisord as well as all the processes it controls are run as
`{{ common_web_user }}`. This allows us to connect to it as that
user which allows us to manage the processes as www-data, instead
of root.

Since xqueue has its own set of dependencies, using a dedicated
virtualenv allows to avoid potential dependencies conflicts when
several services are installed on the same instance.

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