[docs] Install cloudkitty requirements using constraints

The deps configuration for the docs environment only uses upper
constraints for doc/requirements.txt. However, cloudkitty also gets
installed automatically without upper constraints, which produces
various errors on older branches.

Install cloudkitty requirements using constraints to fix this issue.

Change-Id: Ia6f12151be829be6d2f21b3536536697345e05f4
(cherry picked from commit 386f086)

tox.ini

@@ -52,6 +52,7 @@ commands = oslopolicy-sample-generator --config-file=etc/oslo-policy-generator/c
 
 [testenv:docs]
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/yoga}
+       -r{toxinidir}/requirements.txt
        -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W --keep-going -b html doc/source doc/build/html