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: I041acd7c63cf56ec9e90177a82ac0b37e15d4b12

doc/requirements.txt

@@ -3,5 +3,5 @@
 # process, which may cause wedges in the gate later.
 
 openstackdocstheme>=1.18.1 # Apache-2.0
-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

lower-constraints.txt

@@ -69,7 +69,7 @@ rfc3986==0.3.1
 simplejson==3.5.1
 six==1.10.0
 snowballstemmer==1.2.1
-Sphinx==1.6.5
+Sphinx==1.8.0
 sphinxcontrib-websupport==1.0.1
 stevedore==1.20.0
 stestr==2.0.0

tox.ini

@@ -51,8 +51,12 @@ commands =
 
 [testenv:docs]
 basepython = python3
+whitelist_externals =
+  rm
 deps = -r{toxinidir}/doc/requirements.txt
-commands = sphinx-build -b html doc/source doc/build/html
+commands =
+  rm -rf doc/build
+  sphinx-build --keep-going -b html doc/source doc/build/html
 
 [flake8]
 ignore = E123,E126,E128,E241,E265,E713,H202,H405,H238