With the new way of generating docs in the gate[1] our autodoc builds are slightly broken. Put the required dependencies for doc building and autodoc generation into doc/requirements.txt. We can also now remove docs-related requirements from test-requirements.txt. [1] http://lists.openstack.org/pipermail/openstack-dev/2017-December/125710.html Change-Id: I77a09349304451041491893466ca98ba6ebdf96b