Fix modifying a role with same name using LDAP

When the keystone server was configured to use the LDAP assignment
backend and a role was modified with the same name as the role the
operation would fail. This is because the server would check that a
role with the same name existed already and it would of course find
the entry that is currently being modified.

The server is changed to check if the entry currently being modified
is the one with the same name and the operation is allowed if this is
the case.

Change-Id: Ia86ef9fabfd438367a7ed09efb55711860e47d8b
Closes-Bug: #1401721
Merge "Fix modifying a role with same name using LDAP"