Add doc8 a documentation style checking package

Many of the projects are starting to do style checks
on there specs files or other rst files so it seems
appropriate to use a small utility library to do these
kinds of checks (to avoid repeated copied code).

Doc8 starts to create a path for a common rst documentation
checking suite that can avoid the 'test_titles.py' tests
that are appearing in the various specification repositories.

Projects that currently can benefit from this package:

- TaskFlow
- Cinder
- Others with spec repos (nova, glance, oslo...)
 
Change-Id: Ie942497d88ad4dc9a4b3e58bea213354ec5513c5
Merge "Add doc8 a documentation style checking package"