Comment 20 for bug 1199529

I faced the same issue, it looks like it comes from python-heatclient requirement:

horizon requires python-heatclient>=0.2.2
but python-heatclient-0.2.2 requires python-keystoneclient>=0.2,<0.3

updating python-heatclient to 0.2.3 (pip install --upgrade python-heatclient) fixed the problem for me.