Multitenant local mode by default

This PR:

makes local mode multi-tenant by default (chef-zero runs in multi-org mode, providing orgs, acls, groups, etc.), setting chef_server_url = http://localhost:port/organizations/chef
adds the "organization" config parameter
adds the "chef_server_root" config option to specify the top level root
points at Hosted Chef by default when organization is set
infers organization and chef_server_root from chef_server_url when specified, and vice versa
All per opscode/chef-rfc#49