Add pve_exporter deployment steps for Prometheus monitoring

[lae]

Source: https://github.com/znerol/prometheus-pve-exporter

We should be able to:

• create a new local unix user (pve-exp?) to deploy/run under, with random password (needs to be stored on the server somewhere so we can refer to it later, in order to maintain idempotence)
• create a python3 virtualenv
• deploy pve-exporter into that virtualenv
• add PVEAuditor role for pve-exp@pam in PVE auth - probably merge with the associated role variable(s)
• configure pve-exporter to use a pve-exp@pam (?) API user, referencing the password we stored earlier
• create systemd unit file for running pve-exporter, reload on config change
• add note about opening up port 9221 in firewall if using firewall

refer to https://github.com/znerol/prometheus-pve-exporter/wiki/PVE-Exporter-on-Proxmox-VE-Node-using-in-a-virtualenv