p.conf-template.pejacevic: generate json for stretch-pu until EOL

Paul Geversrequested to merge
elbrus/piuparts:develop into develop

For release team support of (old)stable I have set up a britney instance to cover pu-to-stable delta's. As britney has piuparts support, I thought it a good idea to enable it also there. However, https://piuparts.debian.org/stretch-pu/summary.json is missing. I believe this change will enable creation of that file.

Thanks for your work on piuparts and running the service.

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