Add optional spell checker target

Add a tox target using sphinxcontrib-spelling to let
authors run a spell checker against documents before
submitting them.

Based on nova-specs change: If519fb63bc4b0923a48df9e0435125fd66ab5c0c

Change-Id: I5b5bb10fada71c14ae1753e72a450e16d0683cb0

doc/source/conf.py

@@ -30,9 +30,17 @@ extensions = ['sphinx.ext.autodoc',
               'sphinx.ext.intersphinx',
               'sphinx.ext.todo',
               'sphinx.ext.viewcode',
-              'oslosphinx'
+              'oslosphinx',
              ]
 
+# Optionally allow the use of sphinxcontrib.spelling to verify the
+# spelling of the documents.
+try:
+    import sphinxcontrib.spelling
+    extensions.append('sphinxcontrib.spelling')
+except ImportError:
+    pass
+
 todo_include_todos = True
 
 # Add any paths that contain templates here, relative to this directory.

tox.ini

@@ -14,3 +14,10 @@ commands = {posargs}
 
 [testenv:docs]
 commands = python setup.py build_sphinx
+
+[testenv:spelling]
+deps =
+    -r{toxinidir}/requirements.txt
+    sphinxcontrib-spelling
+    PyEnchant
+commands = sphinx-build -b spelling doc/source doc/build/spelling