Python 3: Use distro_python_version for monasca agent CA file

Change-Id: Ia840cd037cd2c2eded429bd0edaede4bb44caa8e
Partially-Implements: blueprint python-3

ansible/roles/monasca/templates/monasca-agent-forwarder/agent-forwarder.yml.j2

@@ -10,7 +10,7 @@ Api:
   project_domain_id: {{ default_project_domain_id }}
   project_domain_name: {{ default_project_domain_name }}
   insecure: False
-  ca_file: /var/lib/kolla/venv/lib/python2.7/site-packages/certifi/cacert.pem
+  ca_file: /var/lib/kolla/venv/lib/python' ~ distro_python_version ~ '/site-packages/certifi/cacert.pem
   max_measurement_buffer_size: {{ monasca_agent_max_buffer_size }}
   backlog_send_rate: {{ monasca_agent_backlog_send_rate }}
   max_batch_size: {{ monasca_agent_max_batch_size }}