Implement doc8 checks for docs

In order to prepare for the move of detailed configuration
content from the install guide to the roles, doc8 checks
are being implemented in the roles to ensure that any new
content submitted is properly checked.

Change-Id: I5cba4c7486738a6588b5ceec24f21b750c71a710
Partially-Implements: blueprint osa-install-guide-overhaul

test-requirements.txt

@@ -4,4 +4,5 @@ flake8>=2.5.4,<2.6.0  # MIT
 # this is required for the docs build jobs
 sphinx>=1.1.2,!=1.2.0,!=1.3b1,<1.3  # BSD
 oslosphinx>=2.5.0,!=3.4.0  # Apache-2.0
-reno>=1.6.2 # Apache-2.0
+doc8 # Apache-2.0
+reno>=1.6.2 # Apache-2.0

tox.ini

@@ -32,9 +32,15 @@ setenv =
 [testenv:docs]
 commands=
     bash -c "rm -rf doc/build"
+    doc8 doc
     python setup.py build_sphinx
 
 
+[doc8]
+# Settings for doc8:
+extensions = .rst
+
+
 [testenv:releasenotes]
 # NOTE(sdague): this target does not use constraints because
 # upstream infra does not yet support it. Once that's fixed, we can