Add the enableVtpm option to created GCE instances

Source for the flag itself:
https://cloud.google.com/compute/docs/instances/modifying-shielded-vm

More information on vTPMs:
https://cloud.google.com/blog/products/gcp/virtual-trusted-platform-module-for-shielded-vms-security-in-plaintext

This option should cost nothing to turn on, and the Cuttlefish team
would like to open the option to use a GCE-provided vTPM as part of some
or all presubmit testing.

Test: `acloud-dev create`, SSH into instance and check for /dev/tpm[rm]0
Change-Id: I1896914709574d3b12bd2b5cef48411c7bb91a09
2 files changed