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. Remove the exists folder before run sphinx-build command

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

Change-Id: I1b1b45bf3ae104defbb16282e18995a28f67f649

doc/requirements.txt

@@ -10,6 +10,5 @@
 mock>=2.0.0 # BSD
 openstackdocstheme>=1.18.1 # Apache-2.0
 reno>=2.5.0 # 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,>=1.6.2;python_version>='3.4'  # BSD
+sphinx>=1.8.0,!=2.1.0 # BSD
 sphinxcontrib-apidoc>=0.2.0  # BSD

lower-constraints.txt

@@ -89,7 +89,7 @@ semantic-version==2.6.0
 simplejson==3.13.2
 six==1.11.0
 snowballstemmer==1.2.1
-Sphinx==1.6.2
+Sphinx==1.8.0
 sphinxcontrib-apidoc===0.2.0
 sphinxcontrib-websupport==1.0.1
 stevedore==1.28.0

tox.ini

@@ -83,11 +83,14 @@ commands =
 
 [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
+  rm -rf doc/build
+  sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
 basepython = python3
@@ -96,7 +99,7 @@ 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:docs-local]
@@ -109,10 +112,13 @@ commands=
 
 [testenv:releasenotes]
 basepython = python3
+whitelist_externals =
+  rm
 deps =
   {[testenv:docs]deps}
 commands =
-  sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
+  rm -rf releasenotes/build
+  sphinx-build -a -E -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]
 basepython = python3