CI: Replace etcd with redis in GATE_IMAGES for cephadm scenario

We replaced redis with etcd in that scenario, but GATE_IMAGES
are not updated.

Change-Id: Ie9d6642f8ce51bc2a35b800c6c149153c14378db

tests/setup_gate.sh

@@ -19,7 +19,7 @@ function prepare_images {
     fi
 
     if [[ $SCENARIO == "cephadm" ]]; then
-        GATE_IMAGES+=",^cinder,^etcd"
+        GATE_IMAGES+=",^cinder,^redis"
     fi
 
     if [[ $SCENARIO == "cells" ]]; then