GKE Shielded VMs

[aumohr]

Describe the bug
In stage 3-gke-multitenant, Shielded VMs are not used for the GKE nodes, even though it is a GCP security best practice.

Rationale
Shielded VMs are virtual machines (VMs) on Google Cloud Platform hardened by a set of security controls that help defend against rootkits and bootkits. Ensures that the boot loader and firmware are signed/untampered. Optionally, if you do not use any custom or unsigned drivers, also enable secure boot.

Expected behavior
Use the Shielded VM option while deploying GKE.

[ludoo]

Reopening as this needs to also be passed in to the nodepool module in node_config.