Unimplemented error on V3 get token

Configuring keystone on V3 with LDAP configured for Assignment and
Identity. When you try to obtain the token (using CURL ie), the keystone
server raises an unimplemented exception. You can see how to reproduce
this on the description's bug.

This is caused because, when you make logging on keystone it assign to
the validate user the 'default' domain (thats because LDAP is
single-domain backend). In the LDAP backend, one of the functions
involved on this workflow is not implemented.

Change-Id: I04faddf888f66978bfe5a330140f8fac9b961a5a
Closes-Bug: #1277463
Merge "Unimplemented error on V3 get token"