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
e578effb33