[CI] Install dependencies for docs target

When tox 'docs' target is called, first it installs the dependencies
(listed in 'deps') in 'installdeps' phase, then it installs nova (with
its requirements) in 'develop-inst' phase. In the latter case 'deps' is
not used so that the constraints defined in 'deps' are not used.
This could lead to failures on stable branches when new packages are
released that break the build. To avoid this, the simplest solution is
to pre-install requirements, i.e. add requirements.txt to 'docs' tox
target.

Conflicts:
  tox.ini

NOTE(elod.illes): conflict is due to branch specific upper constraints
file link.

Change-Id: I4471d4488d336d5af0c23028724c4ce79d6a2031
(cherry picked from commit 494e8d7)
(cherry picked from commit 1ac0d69)

tox.ini

@@ -190,6 +190,7 @@ description =
 # to install (test-)requirements.txt for docs.
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/xena}
+  -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/build/html doc/build/doctrees