tox: Keeping going with docs

Sphinx 1.8 introduced [1] the '--keep-going' argument which, as its name
suggests, keeps the build running when it encounters non-fatal errors.
This is exceptionally useful in avoiding a continuous edit-build loop
when undertaking large doc reworks where multiple errors may be
introduced.

[1] https://github.com/sphinx-doc/sphinx/commit/e3483e9b045

Change-Id: I7d39be84c5c8bd730119220720d2d33cb9e4ef62

doc/requirements.txt

@@ -2,8 +2,8 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 # this is required for the docs build jobs
-sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7' # BSD
-sphinx!=1.6.6,!=1.6.7,>=1.6.2;python_version>='3.4' # BSD
+sphinx>=1.8.0,<2.0.0;python_version=='2.7' # BSD
+sphinx>=1.8.0,!=2.1.0;python_version>='3.4' # BSD
 openstackdocstheme>=1.18.1 # Apache-2.0
 reno>=2.5.0 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD

tox.ini

@@ -30,7 +30,7 @@ whitelist_externals = rm
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
     rm -fr doc/build
-    sphinx-build -W -b html doc/source doc/build/html
+    sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:cover]
 basepython = python3
@@ -48,8 +48,11 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,__init__.py
 
 [testenv:releasenotes]
 basepython = python3
+whitelist_externals = rm
 deps = -r{toxinidir}/doc/requirements.txt
-commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
+commands =
+  rm -rf releasenotes/build
+  sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]
 basepython = python3