Comment 3 for bug 1173117

This is a bugreport which equivalent of blueprint, I think.
However, before filing bluprint, there is a way that can relief this problem.

For example, in case of 'Delete User', flow is here.
1. delete User from Identity
2. delete User from Assignment
3. delete User from Credential
4. delete User from Token

Even just replacing the above procedure of processing the keystone/identity/core.py,
it is possible not to occur a resource orphaned below.