Enable kolla-ansible to use a custom metrics files with Cloudkitty

Cloudkitty has a default (built-in the container) metrics.yml file
 in the /etc/cloudkitty/metrics.yml files. We would like to be able
 to overwrite/customize these metrics configurations via kolla-ansible.
 Cloudkitty is able to use a custom metric file via "metrics_conf".
 Therefore, we are enabling this configuration via Kolla-ansible.

Change-Id: Id9019298482c040be05f540e71dacfdf0bd77469
Signed-off-by: Rafael Weingärtner <rafael@apache.org>