[RFE] Support Credential Encryption Configuration

[Dmitrii Shcherbakov]

https://docs.openstack.org/keystone/pike/admin/identity-credential-encryption.html

A prerequisite for that would be: https://bugs.launchpad.net/charms/+source/keystone/+bug/1514187

This is mostly relevant for non-federated scenarios where password hashes are stored in a SQL database.

With federated identity this is only needed for service accounts.

[Frode Nordahl (fnordahl)]

Initial support for this was added as a part of our implementation of support for Fernet tokens [0].

However, it is worth noting that this only affects the credentials stored in the `credential` table through the use of the Credential API [1].

The passwords for individual users in the directory is already stored using a salted one way hash in the `password` table. [2].

0: https://review.openstack.org/#/q/topic:fernet-keystone-charm+(status:open+OR+status:merged)
1: https://developer.openstack.org/api-ref/identity/v3/#credentials
2: https://github.com/openstack/keystone/blob/d80c260b386dbe6232b1943037739d134855a25e/keystone/common/password_hashing.py