Create-stack-user script should have execute permissions

Currently running stack.sh as root advises you about this script, which is not
executable

This is needed in stable/havana because grenade says to run this
version.

Change-Id: I674af044b8f3c31bcc86be5c6552e8086453d5cd
(cherry picked from commit 105c6e8718da2db50e48cb4a68be8522a80e101e)