Goal: 
Implement application credential support in the Cinder backend driver
to enable zero-downtime password rotation for Glance when using Cinder
as image storage backend.

Currently, the Cinder backend uses hardcoded `V3Password`
authentication, which causes backend operations to fail when
service user passwords are rotated. This story adds support for
`V3ApplicationCredential` authentication, allowing seamless
credential rotation without service disruption.
 

Acceptance Criteria:

• Patch should be ready for upstream review