CHEF-3698: Do not set log_level by default

The default log level for Chef 10 was "info" but on Chef 11 it is "auto"
which does not work on Chef 10. This prevents you from bootstrapping a
Chef 10 node from a Chef 11 workstation. We can leave out log_level and
Chef will use the built in defaults.

https://tickets.opscode.com/browse/CHEF-3698