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

tox.ini

@@ -17,7 +17,9 @@ commands =
   ostestr --concurrency=6 {posargs}
 
 [testenv:pep8]
-commands = flake8
+commands =
+    doc8 doc/source/ CONTRIBUTING.rst HACKING.rst README.rst
+    flake8
 
 [testenv:venv]
 setenv = PYTHONHASHSEED=0
@@ -28,7 +30,9 @@ commands = python setup.py testr --coverage --omit="watcher/tests/*" --testr-arg
 
 [testenv:docs]
 setenv = PYTHONHASHSEED=0
-commands = python setup.py build_sphinx
+commands =
+    doc8 doc/source/ CONTRIBUTING.rst HACKING.rst README.rst
+    python setup.py build_sphinx
 
 [testenv:debug]
 commands = oslo_debug_helper {posargs}
@@ -59,3 +63,8 @@ commands = python setup.py bdist_wheel
 
 [hacking]
 import_exceptions = watcher._i18n
+
+[doc8]
+extension=.rst
+# todo: stop ignoring doc/source/man when https://bugs.launchpad.net/doc8/+bug/1502391 is fixed
+ignore-path=doc/source/image_src,doc/source/man