Use doc8 check

doc8 is a linter for documents and used in openstack-manuals.
It is better to enforce document linters for simple checking.

The current rules are really simple like:
- line length check (80 chars)
- new line at the end of file

Change-Id: I3810df521068ffc97e25a5fc56b72397bf9783bc

tox.ini

@@ -17,13 +17,11 @@ deps =
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
 commands =
-  docs: sphinx-build -W -b html doc/source doc/build/html
   horizon: {envpython} {toxinidir}/manage.py test --settings=horizon.test.settings {posargs}
   manage: {envpython} {toxinidir}/manage.py {posargs}
   py27: {[unit_tests]commands}
   py35: {[unit_tests]commands}
   openstack_dashboard: {envpython} {toxinidir}/manage.py test --settings=openstack_dashboard.test.settings {posargs}
-  releasenotes: sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
   runserver: {envpython} {toxinidir}/manage.py runserver {posargs}
   venv: {posargs}
 
@@ -127,6 +125,16 @@ commands =
   pip install -U {env:TOX_EXTRA_DEPS:}
   {[unit_tests]commands}
 
+[testenv:docs]
+commands =
+  doc8 doc/source
+  sphinx-build -W -b html doc/source doc/build/html
+
+[testenv:releasenotes]
+commands =
+  doc8 releasenotes/source releasenotes/notes
+  sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
+
 [flake8]
 exclude = .venv,.git,.tox,dist,*lib/python*,*egg,build,panel_template,dash_template,local_settings.py,*/local/*,*/test/test_plugins/*,.ropeproject,node_modules
 ignore =
@@ -139,3 +147,14 @@ import-order-style = pep8
 
 [hacking]
 local-check-factory = horizon.hacking.checks.factory
+
+[doc8]
+# File extensions to check
+extensions = .rst, .yaml
+# Maximal line length should be 80 but we have some overlong lines.
+# Let's not get far more in.
+max-line-length = 80
+# Disable some doc8 checks:
+# D000: Check RST validity
+# - cannot handle "none" for code-block directive
+ignore = D000