[CHEF-4725] Ensure IO.sync is set on default log stream

With the switch to setting "default" config values in chef/config.rb
(9c40ec9c), an unconfigured "log_location" setting defaults to
STDOUT, but fails to set STDOUT.sync = true. This results in massive
output buffering under some circumstances, like provisioning through
Vagrant.

This PR should fix all that up!

[CHEF-4725](https://tickets.opscode.com/browse/CHEF-4725)