Declare implementation concept for TenantProvider and -Customizer:

- Provider not to be implemented by consumers
- Customizer to be implemented by consumers

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1451513 13f79535-47bb-0310-9956-ffa450edef68
SLING-2710 Define TenantManager API

- Add service interface as a provider type
  (not to be implemented by consumers)
- Implement TenantManager in the TenantProviderImpl
- Use latest Repository API to leverage repository CRUD
- Move tenant resolution for the AdapterFactory to the
  TenantAdapterFactory (just handing the configuration
  over on construction).

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1451514 13f79535-47bb-0310-9956-ffa450edef68