Preinstall requirements for docs build

Sphinx installs the package with its dependencies, but it does not use
the constraints. This can lead to errors if a dependency introduces any
backward incompatible changes. To avoid this the dependencies needs to
be installed prior to sphinx run by tox with the correct constraints.

Conflicts:
    tox.ini

NOTE(elod.illes): conflict is due to branch specific upper constraints
setting patch.

NOTE(elod.illes): sahara-openstack-ansible-functional job is set as
non-voting as it seems there is a problem in openstack-ansible and the
job fails plus it is also set to non-voting in master until patch
Ib6e09ab83b277704a603c02f0927054478cef8a7 is merged. This is needed to
unblock the gate.

Change-Id: I4ba83f1007dd11801b8e32c6d7566769e005480b
(cherry picked from commit e0136189fc)

.zuul.yaml

@@ -18,7 +18,8 @@
             voting: false
         - sahara-grenade:
             voting: false
-        - sahara-openstack-ansible-functional
+        - sahara-openstack-ansible-functional:
+            voting: false
     gate:
       queue: sahara
       jobs:
@@ -27,7 +28,8 @@
         - sahara-tests-tempest
         - sahara-tests-tempest-v2
         # - sahara-grenade
-        - sahara-openstack-ansible-functional
+        - sahara-openstack-ansible-functional:
+            voting: false
     experimental:
       jobs:
         - sahara-buildimages-ambari

tox.ini

@@ -60,6 +60,7 @@ commands = {posargs}
 [testenv:docs]
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/wallaby}
+  -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands =
     rm -rf doc/html doc/build