Python-heatclient dependency is incorrect for stable/newton

The python-heatclient in setup.py is pinned to 1.5.0, yet the upper constraints file for openstack:

https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt?h=stable/newton

Pins that version to 1.5.1. We should bump to that version in this setup.py.