Move 'check-cherry-picks' test to gate, n-v check

This currently runs in the 'check' pipeline, as part of the pep8 job,
which causes otherwise perfectly valid backports to report as failing
CI. There's no reason a stable core shouldn't be encouraged to review
these patches: we simply want to prevent them *merging* before their
parent(s). Resolve this conflict by moving the check to separate voting
job in the 'gate' pipeline as well as a non-voting job in the 'check'
pipeline to catch more obvious issues.

Change-Id: Id3e4452883f6a3cf44ff58b39ded82e882e28c23
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

tools/check-cherry-picks.sh

@@ -4,11 +4,6 @@
 # to verify that they're all on either master or stable/ branches
 #
 
-# Allow this script to be disabled by a simple env var
-if [ ${DISABLE_CHERRY_PICK_CHECK:-0} -eq 1 ]; then
-    exit 0
-fi
-
 commit_hash=""
 
 # Check if the patch is a merge patch by counting the number of parents.