Add extras directory to prometheus config

This provides a generic mechanism to include extra files
that you can reference in prometheus.yml, for example:

scrape_targets:
  - job_name: ipmi
    params:
      module: default
    scrape_interval: 1m
    scrape_timeout: 30s
    metrics_path: /ipmi
    scheme: http
    file_sd_configs:
    - files:
      - /etc/prometheus/extras/file_sd/ipmi-exporter-targets.yml
      refresh_interval: 5m

Change-Id: Ie2f085204b71725b901a179ee51541f1f383c6fa
Related: blueprint custom-prometheus-targets