Make local mode multi-tenant by default, add organization option

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 https://github.com/opscode/chef-rfc/pull/49