jobs/gerrit-verifications: pipeline: agent none

Don't allocate a slot on one of the jenkins nodes for running the
pipeline. The pipeline job is lightweight, it just starts other jobs
and waits for their completion.

Without this patch we got into a situation where a lot of patches were
submitted to gerrit at the same time, pipelines were started for all of
them, taking up slots, and then all of the pipelines waiting until free
slots become available before starting the actual build jobs. Thus
blocking each other until I manually stopped them.

Related: OS#2385
Change-Id: I289fcf1431a91a38fc37270e6a2f43d744c79e4f

jobs/gerrit-verifications.yml

@@ -384,7 +384,7 @@
     # https://jenkins.osmocom.org/jenkins/scriptApproval/
     dsl: |
       pipeline {{
-        agent {{ label "osmocom-gerrit" }}
+        agent none
         stages {{
           stage("Verification") {{
             parallel {{