Enable image cache

Change-Id: I0f503ac27b9ca0aee09206c0b51e3bb8c7705c9c

run_tests.sh

@@ -325,6 +325,16 @@ EOF
     fi
 )
 
+# enable image cache to avoid downloading images frequently
+(
+    localconf="/opt/stack/new/devstack/local.conf"
+    cat <<EOF >>"$localconf"
+[[post-config|/etc/nova/nova.conf]]
+[xenserver]
+cache_images = all
+EOF
+)
+
 # delete folders to save disk space
 sudo rm -rf /opt/git
 }