Node pool create: add an implicit dependency on parent cluster

[JordanP]

The explicit depends_on is not required as there are other ways
to signal to Terraform that a resource depends on another. Implicit
dependencies are more like TF usually operates.

[adrienthebo]

Note for posterity: The integration CI failure appears spurious:

       1 error(s) occurred:
       
       * module.example.module.gke.local.cluster_master_auth_list_layer2: local.cluster_master_auth_list_layer2: At column 40, line 1: list "local.cluster_master_auth_list_layer1" does not have any elements so cannot determine type. in:
       
       ${local.cluster_master_auth_list_layer1[0]}

[aaron-lane]

@JordanP thank you for the contribution!

[adrienthebo]

@JordanP thanks for making this change! The autogen and root level modules look good, but it doesn't look like this change has propagated to modules/private-cluster. Could you run make generate and see if that updates the private cluster module with the implicit dependency?