missing yaml

Not sure why this just changed but had to fix this in trovestack as
well.

Change-Id: I3ef0666854df3dfc902a85f876fb0e8881622124

scripts/redstack

@@ -792,6 +792,7 @@ function cmd_build_image() {
     sudo $HTTP_PROXY $PKG_MGR $PKG_GET_ARGS update
     sudo $HTTP_PROXY $PKG_MGR $PKG_GET_ARGS install qemu
     sudo -H $HTTP_PROXY pip install --upgrade pip dib-utils
+    pkg_install python-yaml
 
     install_devstack_code