Switch to sphinx-build for doc generation

There is a new Project Testing Interface documentation [1] that
requires projects to use sphinx-build to build their documentation,
instead of the previously used python setup.py build_sphinx. Some
projects have started adapting to this new documentation, including
magnumclient [2].

[1] - http://lists.openstack.org/pipermail/openstack-dev/2017-December/125710.html
[2] - https://review.openstack.org/#/c/554197

Change-Id: I2d464ae432d3019bd82b724722904658f46382cf

python-magnumclient.spec

@@ -181,7 +181,7 @@ Requires:  python3-prettytable
 %py3_build
 %endif
 # generate html docs
-%{__python2} setup.py build_sphinx -b html
+sphinx-build -W -b html doc/source doc/build/html
 # Fix hidden-file-or-dir warnings
 rm -rf doc/build/html/.{doctrees,buildinfo}