Warning log messages about password being truncated upon user update

[Michel Nederlof]

When a user is updated [1] the user object is re-instantiated from the current user object.

In this user dictionary, the password attribute is the hashed password, which can easily be more than the preconfigured 72 chars (when using `bcrypt` method). We have been running keystone since icehouse version, so we still have a lot of passwords that use the `sha512crypt` method (which was the default back then [2]), which are more than 72 chars, easy.

For example:
```
$6$rounds=40000$tgLsnccAfTiJSrKH$/k2pMchhql2t9GbFnHHGdr34v0nTKCgjj3yKct/gv5sSaMeXkUwH641ZVdGV3plOt5JWMgpem6GAtOTlGC.iO1
```

Please note, nothing wrong happens; the password is not actually truncated when saved or updated, as this is handled separately [3].

One way to go about it, is to pop the `password` field from the `old_user_dict`, so the warning only pops up whenever a user really updates it's password.

[1] https://github.com/openstack/keystone/blob/8ca73f758bb613a57815fbe4ae78e3d2afa4af49/keystone/identity/backends/sql.py#L225
[2] https://github.com/openstack/keystone/blob/kilo-eol/keystone/common/utils.py#L127
[3] https://github.com/openstack/keystone/blob/8ca73f758bb613a57815fbe4ae78e3d2afa4af49/keystone/identity/backends/sql.py#L239

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/keystone/+/915256