Share common terraform variables

[msau42]

For example, to use a non-default cluster name, I had to modify the variable in 4 different files.

Looks like terraform itself doesn't support global variables, but there may be a few workarounds:
https://stackoverflow.com/questions/51213871/terraform-provider-variable-sharing-in-modules

[richardsliu]

I think this should be fixed now (#3)

[msau42]

Maybe some files need to be cleaned up, like https://github.com/richardsliu/ray-on-gke/blob/main/platform/modules/kuberay/variables.tf#L21?

I synced up to head yesterday and I found that one of the k8s manifests didn't get deployed. I changed the cluster_name reference in 4 files, but didn't check to see which ones still had the dependency.