update horizon configuration for python3 migrating

Depends-On: https://review.opendev.org/675581
Closes-Bug: #1838719
Partially Implements: blueprint python3-support

Change-Id: Ib8bfb130b8490b583539cc264c2d2a2a034b270c

ansible/roles/horizon/templates/horizon.conf.j2

-{% set python_path = '/usr/share/openstack-dashboard' if horizon_install_type == 'binary' else '/var/lib/kolla/venv/lib/python2.7/site-packages' %}
+{% set python_path = '/usr/share/openstack-dashboard' if horizon_install_type == 'binary' else '/var/lib/kolla/venv/lib/python' + distro_python_version + '/site-packages' %}
+
 Listen {{ api_interface_address }}:{{ horizon_listen_port }}
 
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