Systemd service: improvements

[erebe]

Various improvements over the original config

• Switch from chef-systemd to a template to allow multiple ExecStop
• Start cassandra in foreground to be able to have output in journalctl
• Add timeoutStopExec to avoid that systemd kill "nodetool drain" too early
• remove -h argument in nodetool, as cassandra utility can be only listenning on loopback
• Use nodetool stopdaemon instead of kill $PID
• Suppress exit code of nodetool stopdaemon, so the service appears clean when stopped

[michaelklishin]

@erebe thank you so much!