Remove check_doc.py and use doc8

Instead of using a helper script that is internal to taskflow
use the doc8 package (which was created based on the check_doc.py
work) instead. This avoids repeating duplicated code when a
stackforge/external pypi package can provide the same
functionality.

Change-Id: Ie5a43f96b20e34f3955657ad2ef8beba05a4300e
Merge "Remove check_doc.py and use doc8"