Bug 65145: do not run generate_user_files if stdout is a pipe

pip will run generate_user_files with stdout directed to a log
file, but stdin as a terminal. As such, GUF will wait for user
input, but the user has no idea they should input anything.
This change prevents that.

Change-Id: Iaa596bf40e4d49c73b9a73ee6423c3a6b6899449
Merge "Bug 65145: do not run generate_user_files if stdout is a pipe"