modules/tls/kube/self-signed: Make list explicit
modules/tls/kube/self-signed: Make list explicit

The collection of ids being joined was not explicitly
defined as a list, which is an error in Terraform 0.11.0.

aknuds1/tectonic-installer,coreos/tectonic-installer,coreos/tectonic-installer,aknuds1/tectonic-installer,coreos/tectonic-installer,aknuds1/tectonic-installer,aknuds1/tectonic-installer,aknuds1/tectonic-installer,coreos/tectonic-installer,coreos/tectonic-installer