Documenting ability to control log level during micro_bosh installation

Useful when micro_bosh.deploy displays errors such as 

```
E, [2014-02-18T19:18:19.622573 #6243] [create_vm(bm-14fa2662-766c-4b4c-8901-52b6526bd7c0, urn:vcloud:catalogitem:c2f0ef9b-7a30-44dc-9c69-a96d26f38400, {"ram"=>8192, "disk"=>16384, "cpu"=>4}, ...)] ERROR -- : RestClient exception (retry after 200ms): 400 Bad Request
```