Logging brooklyn-persister thread Jclouds messages in a separate file

When a server with persistence to an object store enabled manages lots of entities the Jclouds log is overwhelmed by messages logged from the `brooklyn-persister` thread (in one case logging at a rate of ~ half a million lines every four minutes).

The appender added in this commit alleviates - a little! - this logging overload by appending messages logged by Jclouds from any thread named "brooklyn-persister" to a separate file.

Anecdotal measurements of the ratio between the persister log and jclouds log sizes, with `EmptySoftwareProcess` entities:
```
0 entities: persister log 0.01 times size of jclouds log
1 entity: 0.36
5 entities: 1.13
10 entities: 1.75
50 entities: 8.6
```

I'm not sure we can set the `persistence` log to `INFO` without doing the same to the `jcloud` log. Maybe we could if we included the Janino library and followed these instructions: http://logback.qos.ch/manual/configuration.html#conditional.