We always use the same directory for terraform fixtures, so it's quite
easy to figure out its path from a pytest fixture by inspecting the
stack. This commit implements this functionality and decreases the
amount of boilerplate needed to write a test.
(Ported from fast)
* add support for internal service account to GKE nodepool module, fixes#62
* update shared vpc example to use internally managed service account
* update shared vpc example to use internally managed service account
* update hub and spoke peering example to use renamed gke nodepool variables