Draft: Monitoring stack w/Prometheus and IPv6

This commit creates a new Monitoring stack option with Prometheus instead of InfluxDB.

A simplified version of the old Monitoring class was created, called MonitoringIPv6. This classes has fewer options (mainly accepts the list of nodes to install the components) and uses the collect facts from ansible to discover the IPv6 address of nodes in Grid'5000 and FIT/IoT-LAB.

By doing so, we're able to monitor the A8 nodes in IoT-LAB infrastructure without using a private VPN in A8 nodes.

Prometheus is used in the place of InfluxDB to have only connection from Grid'5000 to IoT-LAB (incomming IPv6 connections are filtered out in Grid'5000).