Split the creation of $STACK_USER account out of stack.sh

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
Merge "Split the creation of $STACK_USER account out of stack.sh"