Add extend_lists option to merge_yaml

This allows you to extend lists in yaml config. This is useful, for
example, in prometheus.yml, where it would be nice to be able to
extend the scrape_configs to include exporters that aren't packaged
with kolla-ansible. This would provide a mechanism to do so.

Change-Id: I7a10e363f42e8ffaae3c0d2c2a758853e2cab7e1
Related: blueprint custom-prometheus-targets