Deploy Prometheus on WindRiver

Summary:
For FreeBSD, provide a slightly edited service to allow to listen
to internal network.

Scrape targets are dynamically auto-generated from a new pillar
stored in pillar/observability/prometheus.sls. This uses the
services table to resolve name of the services to addresses.

Tests for new prometheus module mocks pillar.get salt method.

Ref T1623

Test Plan:

• New unit tests for _modules/prometheus.py
• Check targets at http://localhost:9090/targets
• Query from Grafana
• Run new NRPE check

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1623

Differential Revision: https://devcentral.nasqueron.org/D3370