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: I0e40eb0e5267d1cdd0ec1195424743310f3c39f8

doc/requirements.txt

@@ -4,5 +4,5 @@
 
 openstackdocstheme>=1.18.1 # Apache-2.0
 reno>=2.5.0 # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,>=1.6.2;python_version>='3.4'  # BSD
+sphinx>=1.8.0,!=2.1.0;python_version>='3.4' # BSD
 sphinxcontrib-apidoc>=0.2.0  # BSD

lower-constraints.txt

@@ -123,7 +123,7 @@ semantic-version==2.6.0
 simplejson==3.13.2
 six==1.10.0
 snowballstemmer==1.2.1
-Sphinx==1.6.2
+Sphinx==1.8.0
 sphinxcontrib-websupport==1.0.1
 sqlalchemy-migrate==0.11.0
 SQLAlchemy==1.2.5

tox.ini

@@ -74,7 +74,7 @@ setenv = PYTHONHASHSEED=0
 deps = -r{toxinidir}/doc/requirements.txt
        -r{toxinidir}/requirements.txt
 commands = rm -rf doc/build doc/source/api
-           sphinx-build -W -b html doc/source doc/build/html
+           sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
 envdir = {toxworkdir}/docs
@@ -82,12 +82,12 @@ deps = {[testenv:docs]deps}
 whitelist_externals =
   make
 commands =
-  sphinx-build -W -b latex doc/source doc/build/pdf
+  sphinx-build -W --keep-going -b latex doc/source doc/build/pdf
   make -C doc/build/pdf
 
 [testenv:releasenotes]
 deps = -r{toxinidir}/doc/requirements.txt
-commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
+commands = sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
 [testenv:bindep]
 # Do not install any requirements. We want this to be fast and work even if