dstat-logger: redirect stdout to /dev/null

The dstat-logger service puts a lot of info into the syslog/journal.
The --output command to write CSV files doesn't appear to suppress the
console output, and I can't see a flag to make it do that.  So
redirect the stdout to /dev/null.

Change-Id: Ib99f8199ebc3c9d89c2b3aa92dff5ff298d03e45

playbooks/roles/dstat-logger/tasks/main.yaml

@@ -18,6 +18,7 @@
       ExecStart=/usr/bin/dstat -tcmndrylpg --tcp --swap --output /var/log/dstat-csv.log
       KillMode=process
       TimeoutStopSec=300
+      StandardOutput=null
       [Install]
       WantedBy=multi-user.target
 - name: Reload systemd