This patch implements the initial support for the
openstack-exporter[0] in the kolla-ansible
prometheus monitoring system.
The configuration and prechecks are reused from the other
exporters and a new template is provided for generating
a os-client-config file required by the exporter.
The default scrape interval is 60 seconds, but it can
be extended via a configuration option.
[0] https://github.com/Linaro/openstack-exporter
Change-Id: I4a34c4bb56e74b5cd544972cbd6540d9acb6e4a1