the variable cluster_tag_key in top-level vars.tf is not used in main.tf

[rberlind]

the variable cluster_tag_key in top-level vars.tf is not used in main.tf. It should be removed, or main.tf should be updated to use it. If it were added to main.tf and passed to the consul-cluster module, the related variable cluster_tag_value should also be added to vars.tf and main.tf and also passed to the consul-cluster module.