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.
:+1:  Having this as a core option will also help 3rd party tools. For example, I already use a config option by this name here:

https://github.com/opscode/knife-opc/blob/master/lib/chef/knife/opc_org_create.rb#L46