Reconfigure overseer to write logs to /tmp

docker/supervisord-lvalert-overseer.conf

 [program:overseer]
 command=lvalert_overseer -a %(ENV_LVALERT_USER)s -b %(ENV_LVALERT_PASSWORD)s
     -s %(ENV_LVALERT_SERVER)s -p %(ENV_LVALERT_OVERSEER_PORT)s
-    -l /dev/stdout -e /dev/stderr
+    -l /tmp/overseer_stdout.log -e /tmp/overseer_stderr.log
 user=gracedb
 group=www-data
 stdout_logfile=/dev/stdout