Automatically creating a new user account is not always the right course
of action when stack.sh is running as root. Plus, the re-exec did not
work correctly in some cases.
* Create tools/create-stack-user.sh to set up a suitable user
for running DevStack
* Abort stack.sh and unstack.sh if running as root and suggest creating a
suitable user account.
Change-Id: I5d967c00c89f32e861449234ea8fe19261cd9ae3
