Added doc8

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

test-requirements.txt

@@ -4,6 +4,7 @@
 
 coverage>=3.6
 discover
+doc8 # Apache-2.0
 hacking>=0.10.2,<0.11
 mock>=1.2
 oslotest>=1.10.0  # Apache-2.0
@@ -19,4 +20,5 @@ sphinx>=1.1.2,!=1.2.0,!=1.3b1,<1.3
 sphinxcontrib-pecanwsme>=0.8
 
 # For PyPI distribution
-twine
+twine
+