Adds MON Prometheus exporter

Adds a new MON component called mon-exporter, which collects
vdu infra metrics and exposes them through a webservice
by using prometheus_client. This webservice follows the Prometheus
exporter format so it can be integrated as a target so that
it polls metrics from there.

Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>
Change-Id: Iaf2073879d884d0597aa8341f7b6fbbbc3d86e7e