Add mechanism to scrape out of tree prometheus exporters

kolla-ansible has the capability to deploy prometheus. At present is only configured to scrape the exporters that kolla-ansible deploys. Rather than add more exporters to the kolla and kolla-ansible source trees, it would be preferable to be able to deploy the exporters via some other mechanism and configure the kolla-ansible deployed prometheus to scrape them. Several services also have built in exporters which also don't need to be deployed separately e.g ceph. Two changes are required to make this work:

- Add a mechanism to supplement the kolla-ansible prometheus config
- Allow extra files to be mounted into the container which can be referenced by prometheus config.For
  example, this is necessary to configure file based service discovery.