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: I2858ce6bf73471328b92b52be1cdc18ab46254d9

doc/requirements.txt

@@ -3,8 +3,8 @@
 # process, which may cause wedges in the gate later.
 # These are needed for docs generation
 openstackdocstheme>=1.20.0 # 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,!=2.1.0,>=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
 reno>=2.5.0 # Apache-2.0
 
 mock>=2.0.0 # BSD

lower-constraints.txt

@@ -38,7 +38,7 @@ requestsexceptions==1.2.0
 six==1.10.0
 smmap==0.9.0
 snowballstemmer==1.2.1
-Sphinx==1.6.2
+Sphinx==1.8.0
 sphinxcontrib-websupport==1.0.1
 stestr==2.0.0
 stevedore==1.20.0

tox.ini

@@ -45,8 +45,11 @@ commands = {posargs}
 
 [testenv:docs]
 basepython = python3
+whitelist_externals = rm
 deps = -r{toxinidir}/doc/requirements.txt
-commands = sphinx-build -W -b html doc/source doc/build/html
+commands =
+  rm -fr doc/build
+  sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [flake8]
 # E731 skipped as assign a lambda expression
@@ -65,8 +68,11 @@ commands = python3 benchmark/benchmark.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