Validate name for worker_node_pools.

[DeprecatedLuke]

The name field for the worker_node_pools is not validated which will result in:

E0527 21:53:30.089025       1 hetzner_node_group.go:119] failed to create error: could not create server type cpx21 in region fsn1: invalid input in field 'name' (invalid_input)

(I used _'s in this case)

The same logic that is used for the cluster_name field should be applied to the worker_node_pools name field.