Alert when Cinder pool aggregate ID is missing

[hemna]

Summary

• Add a Cinder alert for pools without an aggregate ID.
• Include shard, backend, and pool in the alert metadata and annotations.

Dependency

• Depends on Add Cinder pool aggregate ID missing metric openstack-exporter#34, which adds the cinder_pool_aggregate_id_missing metric.

Verification

• helm lint . -f ci/test-values.yaml
• helm template openstack-exporter . -f ci/test-values.yaml --set openstack.alerts.enabled=true
• git diff --check