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: Idec07bdb441a2f30351267686f2f74888655a6df

doc/requirements.txt

@@ -2,7 +2,7 @@
 # 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

tox.ini

@@ -17,14 +17,18 @@ commands = {posargs}
 
 [testenv:docs]
 basepython = python3
+whitelist_externals =
+  rm
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
-commands = sphinx-build -W -b html doc/source doc/build/html
+commands =
+  rm -rf doc/build
+  sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:spelling]
 deps =
    -r{toxinidir}/requirements.txt
    sphinxcontrib-spelling
    PyEnchant
-commands = sphinx-build -b spelling doc/source doc/build/spelling
+commands = sphinx-build --keep-going -b spelling doc/source doc/build/spelling