Remove DEFAULT_STACK_USER, just use STACK_USER

STACK_USER needs to be set for more than just stack.sh, there
was no real distinction for using DEFAULT_STACK_USER instead
of just setting STACK_USER directly in stackrc and allowing it
to be overridden in localrc.

Change-Id: I9e8d70db29bb421f1ce3dbf40a5ad299cc7ea785
Merge "Remove DEFAULT_STACK_USER, just use STACK_USER"