Allow delete user or group at same time as role

If a user was deleted at the same time as a role that the user has
been granted the server may respond with a 404 Not Found because the
user doesn't exist when the Keystone server checks for it.
Whether there's an error or not depends on timing.

Also, if the system is configured to use LDAP as the identity
backend, the user may be removed directly from the LDAP server
while the role still exists. The role should be able to be deleted
and not get a 404 Not Found.

(The description mentioned users, but this also applies to groups.)

The server is changed so that it doesn't check for the existence
of the user or group.

Related to bp use-common-oslo-db-code

Change-Id: If59a84a4f57d7be1d2639d0a2f0415e20292dfc3
Merge "Allow delete user or group at same time as role"