Add multitenancy support to Chef client config and local mode

This adds multitenant chef-zero support to local mode by default, and adds options to make multitenancy and Hosted Chef much easier to develop for and work with.
Can we be explicit about what the default value of chef_server_root will be?  I am assuming that if chef_server_url is set then the default would be something like this:

```
chef_server_url.gsub(/\/organizations\/+[^\/]+\/*$/, '')
```

although for the RFC it is probably better to specify it via example than the above code.