inject default values to from_config_file method

[guskovd]

for cases where there are no corresponding fields in the file

eg
modern "/etc/chef/client.rb" files like:

chef_server_url "_"
client_fork true
log_location '/var/log/chef/client.log'
no_lazy_load true
validation_client_name "chef-validator"
verify_api_cert true
environment "testing"
node_name "_"

...

[coderanger]

Not sure what the goal is here. I've kept the from_config_file API thin so you just get what was in the file and that's it. If you want to do overrides, do them in your own code afterwards.