[update] Allow overriding NodeSets for compute reboot

[srac0]

Make the NodeSet fetch in reboot_hypervisor_using_cr.yml conditional on cifmw_update_node_sets being undefined. This allows callers to pre-set the variable to control which NodeSets are targeted for reboot instead of always fetching all NodeSets in the namespace.

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https://gateway-cloud-softwarefactory.apps.ocp.cloud.ci.centos.org/zuul/t/rdoproject.org/buildset/4cd9fa4657d04703a16d8a5e59ff7185

✔️ openstack-k8s-operators-content-provider SUCCESS in 3h 02m 40s
❌ podified-multinode-edpm-deployment-crc POST_FAILURE in 22m 13s
✔️ cifmw-crc-podified-edpm-baremetal SUCCESS in 1h 52m 09s
✔️ cifmw-crc-podified-edpm-baremetal-minor-update SUCCESS in 2h 35m 18s
✔️ cifmw-pod-zuul-files SUCCESS in 5m 15s
✔️ openstack-k8s-operators-content-provider-bootc SUCCESS in 2h 38m 03s
✔️ cifmw-crc-podified-edpm-baremetal-bootc SUCCESS in 1h 52m 31s
✔️ noop SUCCESS in 0s
✔️ cifmw-pod-ansible-test SUCCESS in 9m 14s
✔️ cifmw-pod-pre-commit SUCCESS in 9m 33s
✔️ cifmw-molecule-update SUCCESS in 5m 24s