repo-install-test: add label for relevant nodes

Add the slave axis again, so the jobs aren't stuck forever. I had
assumed that without the axis, it would run on any node, but that's not
how it works. Add a label for this job, with several nodes attached,
like we do it for TTCN-3, master-builds, gerrit-verifications etc.

Related: https://jenkins.osmocom.org/jenkins/label/repo-install-test/
Related: OS#4567
Fixes: fcf669 ("repo-install-test: run on all build slaves again")
Change-Id: I276ab47f76a0f4db542ca99825ebb019236b4d27

jobs/repo-install-test.yml

@@ -39,6 +39,10 @@
                 docker-playground.git branch
           default: 'master'
     axes:
+      - axis:
+          type: slave
+          name: label
+          values: !!python/tuple [repo-install-test]
       - axis:
           type: user-defined
           name: feed