fix: update upper constraints URL to use stackhpc fork

Author: jackhodgkiss

Dockerfile

@@ -46,7 +46,7 @@ ARG TEMPEST_PLUGIN_NEUTRON_VERSION=master
 ARG RALLY_OPENSTACK_SOURCE=https://github.com/stackhpc/rally-openstack.git
 # Update after https://github.com/stackhpc/rally-openstack/pull/3/files has been merged.
 ARG RALLY_OPENSTACK_VERSION=non-admin-credentials
-ARG RALLY_OPENSTACK_UPPER_CONSTRAINTS=https://raw.githubusercontent.com/openstack/rally-openstack/$RALLY_OPENSTACK_VERSION/upper-constraints.txt
+ARG RALLY_OPENSTACK_UPPER_CONSTRAINTS=https://raw.githubusercontent.com/stackhpc/rally-openstack/$RALLY_OPENSTACK_VERSION/upper-constraints.txt
 
 RUN apt-get update && apt-get install --yes sudo python3-dev python3-pip vim git-core crudini jq iputils-ping && \
     apt clean && \