Simple centralized logging for small Debian-based cluster

Question

I have small cluster with 8 servers. Every one has Debian 8.4 installed. I'm only one administrator. What would be the simplest solution for centralized logging with following requirements:

1. Easy to install and operate (I'm only one administrator).
2. Support logs from systemd services.
3. Support logs for base services (NFSd, cron, dmesg, etc).
4. Support file logs from applications.
5. Web interface for querying logs with alerting capabilities.

Thank you!