Implement centralized logging

Fix #263 (closed)

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Note: I only deployed the loki and promtail role and the Grafana and Nginx changes were deployed by hand. Someone with vault access needs to add vault_loki_nginx_user and vault_loki_nginx_passwd (I used tr -dc [:alnum:] < /dev/urandom | head -c 32).

Note2: This isn't really tested (as always), so deploying before merge is probably a good idea.