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: I2831ab17d943638062b709882c7a1cc481e46476

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.
 
-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
 reno>=2.5.0
 openstackdocstheme>=1.18.1

lower-constraints.txt

@@ -5,6 +5,6 @@ enum34==1.0.4
 PrettyTable==0.7.1
 oslotest==1.5.1
 stestr==2.0.0
-sphinx==1.6.2
+Sphinx==1.8.0
 reno==2.5.0
 openstackdocstheme==1.18.1

tox.ini

@@ -46,7 +46,7 @@ deps =
   -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/build
-  sphinx-build -W -b html doc/source doc/build/html
+  sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:debug]
 basepython = python3
@@ -59,7 +59,11 @@ deps =
   -r{toxinidir}/requirements.txt
 
 [testenv:releasenotes]
-commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
+whitelist_externals =
+  rm
+commands =
+  rm -rf releasenotes/build
+  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.