Do not hardcode "unstable" as development repository

Instead use getattr(settings, 'DISTRO_TRACKER_DEVEL_REPOSITORIES')[0]
3 jobs for kali-debci in 7 minutes (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #167833
flake8

00:03:44

passed #167835
functional-tests

00:07:00

passed #167834
unit-tests

00:05:48

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