refactor: use GetKeystoneAPIByName instead of label-based lookup

[ratailor]

• Replace GetKeystoneAPI with GetKeystoneAPIByName in nova_controller.go
• use instance.Spec.KeystoneInstance directly instead of empty label map
• Remove TODO comment about changing the API usage.
• Update keystone-operator/api dependency to support new functionality

This provides a more direct and efficient way to get the KeystoneAPI resource by name rather then searching by labels.

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✔️ nova-operator-kuttl SUCCESS in 43m 57s
✔️ nova-operator-tempest-multinode SUCCESS in 2h 12m 49s
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Build failed (check pipeline). Post recheck (without leading slash)
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you rerun jobs.

https://softwarefactory-project.io/zuul/t/rdoproject.org/buildset/0a37fc843d32461ba235d449e3a82130

✔️ openstack-meta-content-provider SUCCESS in 2h 59m 48s
✔️ nova-operator-kuttl SUCCESS in 36m 56s
✔️ nova-operator-tempest-multinode SUCCESS in 2h 34m 22s
❌ nova-operator-tempest-multinode-ceph FAILURE in 49m 15s