Tox entry point to lint docstrings

pep257 define a standard for docstrings.  The linter is available as a
flake8 extension 'flake8-docstrings'.

Hack in the flake8 command to only include errors starting with D.

Usage:

  tox -e flake8-docstrings

Change-Id: If8b5788aee1e64db8accead39c3058057874932b
Merge "Tox entry point to lint docstrings"