For a better doc QoS, we now use doc8 as part of the testing
procedure while removing the existing tests we had on doc formatting.
I also updated tox.ini to run doc8 as within 'pep8' and 'docs' venvs.
Change-Id: Ia0ad99541509f4c026e26d28c41ff0210b12a504
Closes-Bug: #1524228