This patch change the log format from:

```
2016-03-25 04:26:37,609 WARNING access 172.17.0.2 - Anonymous [25/Mar/2016:04:26:37 +0200] "GET / HTTP/1.0" 200 0 "" ""
------
2016-03-25 04:26:37,613 WARNING access 172.17.0.2 - Anonymous [25/Mar/2016:04:26:37 +0200] "GET / HTTP/1.0" 200 1971 "" ""
------
2016-03-25 04:26:37,619 WARNING access 172.17.0.2 - Anonymous [25/Mar/2016:04:26:37 +0200] "GET / HTTP/1.1" 200 1952 "" ""
------
2016-03-25 04:26:40,619 WARNING access 172.17.0.2 - Anonymous [25/Mar/2016:04:26:40 +0200] "GET / HTTP/1.0" 200 1971 "" ""
------
2016-03-25 04:26:43,622 WARNING access 172.17.0.2 - Anonymous [25/Mar/2016:04:26:43 +0200] "GET / HTTP/1.0" 200 1971 "" ""
```

to something more concise for the eye:


```
2016-04-06 10:13:41,854 WARNING access 172.17.0.2 - Anonymous [06/Apr/2016:10:13:41 +0200] "GET / HTTP/1.0" 200 1971 "" ""
2016-04-06 10:13:44,861 WARNING access 172.17.0.2 - Anonymous [06/Apr/2016:10:13:44 +0200] "GET / HTTP/1.0" 200 1971 "" ""
2016-04-06 10:13:47,868 WARNING access 172.17.0.2 - Anonymous [06/Apr/2016:10:13:47 +0200] "GET / HTTP/1.0" 200 1971 "" ""
2016-04-06 10:13:50,875 WARNING access 172.17.0.2 - Anonymous [06/Apr/2016:10:13:50 +0200] "GET / HTTP/1.0" 200 1971 "" ""
2016-04-06 10:13:53,882 WARNING access 172.17.0.2 - Anonymous [06/Apr/2016:10:13:53 +0200] "GET / HTTP/1.0" 200 1971 "" ""
2016-04-06 10:13:56,890 WARNING access 172.17.0.2 - Anonymous [06/Apr/2016:10:13:56 +0200] "GET / HTTP/1.0" 200 1971 "" ""
```

by changing the log format from the default `------\n%(asctime)s %(levelname)s %(name)s %(message)s` ( defined [here](https://github.com/zopefoundation/ZConfig/blob/0d6a8a1/ZConfig/components/logger/handlers.xml#L38
)) to simply `%(asctime)s %(levelname)s %(name)s %(message)s`

It is enabled for both event.log and Z2.log


/cc @kazuhiko @vpelletier @tc 


/reviewed-on nexedi/slapos!67