multi-backend support for identity

The current code has a number of problems and limitations in its
support for having domain-specific backends (e.g. a different LDAP
server per domain).  Not least of the problems is that you cannot
always infer the domain if an API call is just handed a user_id or
group_id. These issues are so severe that this feature is currently
marked as experimental.

This patch fixes these issues by using a mapping layer to store
the domain and local ID for the public facing user and group IDs.
No API changes are required for this new support. An important
consequence of this change is that non-UUID IDs for backends
like LDAP do not escape from keystone.

To ensure backward compatibility with existing single backend
installations, the mapping is not used for the default driver.
An exception to this is that if a cloud provider wants to enable
mapping for the default LDAP driver then they can set a config
option to achieve this.

keystone-manage has been extended to provide options to purge
the mapping table.

Blueprint: multi-backend-uuids

Change-Id: I60f8965bb74b248e6a6c8f141289affa431ee3cf
Merge "multi-backend support for identity"