tools: Allow check-cherry-picks.sh to be disabled by an env var

The checks performed by this script aren't always useful to downstream
consumers of the repo so allow them to disable the script without having
to make changes to tox.ini.

Change-Id: I4f551dc4b57905cab8aa005c5680223ad1b57639

Author: lyarwood

tools/check-cherry-picks.sh

@@ -4,6 +4,11 @@
 # 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.