Add mon-exporter as scrape target in Prometheus

The Prometheus sidecar is expected to generate the scrape targets from
the config map and from the prometheus_jobs in MongoDB. MON is adding
mon-exporter as prometheus_job in MongoDB. There are race conditions in
the start of MON and Prometheus that prevent the sidecar to be aware
of the mon-exporter being added to prometheus_jobs collection in
MongoDB.

This change adds mon-exporter to the default scrape-targets and reconciles
the jobs in Mongo and the jobs in the configmap, prevailing the job in the
configmap if duplicated.

Change-Id: Ib6f1cb89723d7b4942eeb77fd0ea46a0d3493d58
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>