Update docs to use sphinx-build

Update the tox.ini helper env 'docs' to use sphinx-build directly like
the job does.

Use the build-sphinx-docs job rather than the openstack version as we do
not need constraints in zuul.

This does not move doc requirements to doc/requirements.txt because the
Zuul docs document the tests too, so the doc build needs all the things
in test-requirements as well.

Change-Id: Id81c05725d4dbe12dccc0272c396c92ff2f475ec
diff --git a/.zuul.yaml b/.zuul.yaml
index c820c8e..8e69ec5 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -32,7 +32,7 @@
     name: openstack-infra/zuul
     check:
       jobs:
-        - build-openstack-sphinx-docs:
+        - build-sphinx-docs:
             irrelevant-files:
               - zuul/cmd/migrate.py
               - playbooks/zuul-migrate/.*
@@ -46,7 +46,7 @@
         - zuul-stream-functional
     gate:
       jobs:
-        - build-openstack-sphinx-docs:
+        - build-sphinx-docs:
             irrelevant-files:
               - zuul/cmd/migrate.py
               - playbooks/zuul-migrate/.*