Make use of STACK_USER instead of relying on USER

Quite easily one ends up calling ./stack.sh in an environment that,
albeit being user "stack" (for example), doesn't quite meet the
expectations of devstack.  The errors that follow can be rather hard to
track down, as the dependency on `USER` is not mentioned.

To remedy this situation, this commit

- uses STACK_USER instead of USER and
- mentions that dependency in the script headers of lib/*

Change-Id: If4cdc39b922ea64b4c0893a0e695ec06349fccc5
Merge "Make use of STACK_USER instead of relying on USER"