This is a squash of two commits that need to be merged together in order
to get all tests to pass:

Commit message #1:

Use Zuul v3 fetch-subunit-output

We have consolidated the fetch output roles into one
fetch-subunit-output, replace useage of old roles with new one.

Original change-id: Iae2892d9b4cd870a11579434edc9ee66bd16798c
(cherry picked from commit 0a2ee90)

Commit message #2:

Fix sphinx-docs job for stable branch

build-openstack-sphinx-docs jobs fail on stable branches because the
new pip version 10 changed a Warning into an Error: When calling
"pip install" without any package name, the command now fails.
tox_install.sh is being called during docs job without any package
passed to pip.

Note that this only affects stable/pike and older, so there is no patch
in master to cherry-pick this from. Newer branches use a different way
to set up the tox environment.

Depends-On: https://review.openstack.org/573665
Change-Id: I35e9ede7103031ea3ea45e35d743e63059fb8d0b