Improve Tenant:

- Clarify getName() and getDescription() if properties are missing
- Add unit tests
- Allow TenantImpl to reload properties
- Declare Tenant as provider type interface (not to be implemented
   by consumers)

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1451511 13f79535-47bb-0310-9956-ffa450edef68