This commit follows up the work in Kolla to provide deploy and configure the
Prometheus blackbox exporter.

An example blackbox-exporter module has been added (disabled by default)
called os_endpoint. This allows for the probing of endpoints over HTTP
and HTTPS. This can be used to monitor that OpenStack endpoints return a status
code of either 200 or 300, and the word 'versions' in the payload.

This change introduces a new variable `prometheus_blackbox_exporter_endpoints`.
Currently no defaults are specified because the configuration is heavily
dependent on the deployment.

Co-authored-by: Jack Heskett <Jack.Heskett@gresearch.co.uk>
Change-Id: I36ad4961078d90e2fd70c9a3368f5157d6fd89cd