enable prometheus-openstack-exporter to use ca cert

you might refer to:
https://github.com/gophercloud/utils/blob/b0167b94122ca14ce50258a760b4e9b22788f0d7/openstack/clientconfig/results.go#L41

Change-Id: Ia326360c412aad9ca4d1735cc6486aa2fce22c1a
Closes-Bug: #1850812

ansible/roles/prometheus/templates/clouds.yml.j2

@@ -10,3 +10,4 @@ clouds:
      project_domain_name: 'Default'
      user_domain_name: 'Default'
      auth_url: {{ admin_protocol }}://{{ kolla_internal_fqdn | put_address_in_context('url') }}:{{ keystone_admin_port }}/v3
+     cacert: {{ openstack_cacert }}

releasenotes/notes/bug-1850812-e840453d44e17d34.yaml

+---
+fixes:
+  - |
+    Fix prometheus-openstack-exporter to use CA certificate.