tox: Keeping going with docs

1. 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.

2. Cleanup the folders before run commands

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

Change-Id: Ib3133820a3d3ca3027ea2f9bc9b6907929cff8ed

doc/requirements.txt

@@ -4,6 +4,6 @@
 
 openstackdocstheme>=1.19.0 # Apache-2.0
 reno!=2.3.1,>=1.8.0 # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7'  # BSD
+sphinx>=1.8.0,<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,!=2.1.0;python_version>='3.4' # BSD
 sphinxcontrib-svg2pdfconverter>=0.1.0 # BSD

tox.ini

@@ -116,14 +116,17 @@ deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/doc/requirements.txt
-commands = sphinx-build -W -b html doc/source doc/build/html
+commands =
+  rm -rf doc/build/html
+  sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
 basepython = python3
 whitelist_externals = make
 deps = {[testenv:docs]deps}
 commands =
-  sphinx-build -W -b latex doc/source doc/build/pdf
+  rm -rf doc/build/pdf
+  sphinx-build -W --keep-going -b latex doc/source doc/build/pdf
   make -C doc/build/pdf
 
 [testenv:debug]
@@ -138,7 +141,7 @@ deps =
     -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf releasenotes/build
-  sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
+  sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
 [flake8]
 # E123, E125 skipped as they are invalid PEP-8.