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
Merge "Add optional spell checker target"