WIP: feat: add prometheus role

[Fissium]

Add two new roles to deploy and configure Prometheus exporters:

• node_exporter
• postgres_exporter

[vitabaks]

I myself prefer my version of pgwatch instead of the prometheus exporter, but maybe it will be useful to someone, thanks for the contribution.

I suggest moving the variables to main.yml under Netdata (monitoring section).

[vitabaks]

It may be worth combining the tasks into one "prometeus" role.

[Fissium]

Argee

[Fissium]

@vitabaks I’ve refactored the PR—could you take a look and let me know if everything’s good now?

[vitabaks]

@Fissium Please note that refactoring has been performed #992 (for migration to the collection structure) and now all variables are in the roles/common/defaults/main.yml file.