Set kubeconfig and asset_dist as sensitive

* Mark `kubeconfig` and `asset_dist` as `sensitive` to
prevent the Terraform CLI displaying these values, esp.
for CI systems
* In particular, external tools or tfvars style uses (not
recommended) reportedly display all outputs and are improved
by setting sensitive
* For Terraform v0.14, outputs referencing sensitive fields
must also be annotated as sensitive

Closes https://github.com/poseidon/typhoon/issues/884

bare-metal/fedora-coreos/kubernetes/outputs.tf

@@ -1,10 +1,12 @@
 output "kubeconfig-admin" {
-  value = module.bootstrap.kubeconfig-admin
+  value     = module.bootstrap.kubeconfig-admin
+  sensitive = true
 }
 
 # Outputs for debug
 
 output "assets_dist" {
-  value = module.bootstrap.assets_dist
+  value     = module.bootstrap.assets_dist
+  sensitive = true
 }