Copy Exim logs in system-config-run jobs

It's good to be able to look at the MTA logs and see whether
anything's (attempted to be) sent, since we block SMTP egress from
these test nodes by default.

Change-Id: I02154f2b1b6cfdf1c3914d3877c80c9289057057

zuul.d/system-config-run.yaml

@@ -15,6 +15,7 @@
       copy_output:
         '/var/log/syslog': logs_txt
         '/var/log/messages': logs_txt
+        '/var/log/exim4': logs
         '/var/log/docker': logs
         '/var/log/containers': logs
         '/var/log/dstat-csv.log': logs