Fix Terraform plugin cache, bastion host login user

[Nuru]

what

1. Change location of Terraform plugin cache from $HOME/.terraform.d/plugins to $HOME/.terraform.d/plugin-cache
2. [kopsctl] Change bastion host login user from admin to ubuntu
3. Restore KUBECONFIG environment variable
4. Ensure $KUBCONFIG exists with no group or world read permissions
5. Attempt to disable dependabot PRs

why

1. $HOME/.terraform.d/plugins is an implied Terraform mirror directory and using it as a plugin cache breaks both the cache and the mirror
2. kops switched default OS from Debian (with default user admin) to Ubuntu (with default user ubuntu)
3. Kubernetes prompt from kube-ps1 is disabled if $KUBECONFIG is empty at startup, and kubectl complains if $KUBECONFIG does not exist
4. kubectl complains if $KUBECONFIG is group or world readable
5. Not sure exactly why or how dependabot PRs were enabled, but they duplicate the PRs created by Renovate, so they add unnecessary noise

notes

Officially, $KUBECONFIG is a list of possible files to use for configuration, but Geodesic assumes in many places that it points to only a single file. (Issue #457)

references

• Terraform plugin cache and implied mirror directories