Bump Openstack Exporter version to 0.6.0

There appears to be a bug in the prometheus openstack exporter versions
<= 0.2.1 causing cinder data to stop being collected after about 24
hours.

Bumping to latest stable version 0.6.0.

Change-Id: Iccaf4c0bb79e7e33072e37aa83711b37938d6719
Signed-off-by: Keith Plant <[email protected]>
(cherry picked from commit cd7f78e)

Author: keithpl

docker/prometheus/prometheus-openstack-exporter/Dockerfile.j2

@@ -4,7 +4,7 @@ LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build
 {% block prometheus_openstack_exporter_header %}{% endblock %}
 
 {% block prometheus_openstack_exporter_repository_version %}
-ENV prometheus_openstack_exporter_version=0.2.1
+ENV prometheus_openstack_exporter_version=0.6.0
 {% endblock %}
 
 {% block prometheus_openstack_exporter_install %}