Remove Default Tenant

Default tenant is an ambiguous construct. It contains an _implied_ role which is not called out explicitly. This blueprint suggests getting rid of a default tenant and, where the functionality is needed, allowing various implementations to represent the relationship using a role (ex. Dashboard is going to create a "Member" role which will be used to associate users with tenants).

Impact:
1. Authenticating without a tenant specified will return an unscoped token (as per original design spec here: https://github.com/rackspace/keystone/raw/master/doc/design/flow_diagram.png).
2. Identity managers using the service API and back-end plug-in developers will need to manage the link between user and tenant.
3. Some APIs will change (like PUT /users/userId/tenant).