A non-existing tenant can be added to a user

1. Create a user without setting the tenantId.

2. Update this user with adding a non-existing tenantId property
http://10.1.0.99:35357/v2.0/users/354eb97979364a368602b7ebf6362617
{
    "user": {
    "name": "user1",
    "email": "<email address hidden>",
    "tenantId":"55555555555555",
    "pass": "passw0rd",
    "enabled": true
    }
}

Actual Results: The user gets updated with the non-existing tenantId successfully.
[root@vs10 ˜]# keystone user-get user1
+----------+----------------------------------+
| Property | Value |
+----------+----------------------------------+
| email | <email address hidden> |
| enabled | True |
| id | 354eb97979364a368602b7ebf6362617 |
| name | user1 |
| pass | passw0rd |
| tenantId | 55555555555555 |
+----------+----------------------------------+

Expected Results: Updating the user with the non-existing tenantId should fail.

Note that if the user already has a tenantId assigned to it, updating with a non-existing tenantId will fail as expected.