Fix behavior of build_image --pull (Fixes#5640)
Also refactor helper.py:
1. Change behavior of functions so that most return True on success and False on failure.
2. Only main will return 1 on failure and 0 on success now. Previous behavior was very error prone.
3. Rename _get_output_dir to _get_out_dir.
4. Make function docstrings use descriptive tense.
5. Make helper.py print help when no argument is specified.