Pin python-novaclient to fix API test

Change-Id: Iaa1c74e92cf00aae71cc7c5bca9caa545adbd341

tox.ini

@@ -17,7 +17,8 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONDONTWRITEBYTECODE=1
 deps = -r{toxinidir}/requirements/requirements.txt
        -r{toxinidir}/tests/test-requirements.txt
-commands = pip install -U mock==1.0.1
+commands = pip install python-novaclient==2.26.0
+           pip install -U mock==1.0.1
            pip install git+https://github.com/stackforge/opencafe.git#egg=cafe
            nosetests {posargs:--exclude=api --exclude=endtoend --nologcapture}