Changes

jenkins-common.sh: add d9 to upstream_distro func

After the default UPSTREAM_DISTRO was changed from debian:stretch to
debian:bullseye, the "debian9-repo-install-test" container has gotten
built with bullseye instead of stretch. This is the reason for failures
of the jenkins job Osmocom-repo-install-debian9 we have seen over the
past days.

With this patch applied, it runs through again:
https://jenkins.osmocom.org/jenkins/job/Osmocom-repo-install-debian9/339/

Change-Id: I98a19184ba936114c03cd5cc4f54a3173cbd9cfe