Feature 10961 Addition of external targets to OSM Prometheus

This commit allows the automatic regeneration of a Prometheus config
file from a sidecar by combining the information in PROMETHEUS_BASE_CONFIG_FILE
and the prometheus_jobs in mongo.

Change-Id: I0246d60ec32733a088ca8590a6445735549bf4de
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>