fix(2-networking-a-fedramp): encrypt NVA boot disk with the vdss KMS key

[AloysJehwin]

Fixes #193

The FedRAMP High/Moderate networking stage builds a vdss-keyring HSM key and grants roles/cloudkms.cryptoKeyEncrypterDecrypter to both the NVA service account and the VDSS host project's compute service agent — and then nothing ever uses it. The NVA boot disks come up Google-managed, so those grants have no effect and the appliance disks aren't CMEK-encrypted.

The IL5 stage next door creates the identically-named key and actually wires it up, on the bastion disks and the NGFW VMs. This just brings the a-fedramp stage in line by passing the key to the NVA template.

I used the same top-level encryption block the IL5 bastion uses rather than reaching into boot_disk, since that's the module's supported surface. Worth noting the NVA sets create_template = true, so I checked the template path specifically — compute-vm's template disk block emits disk_encryption_key { kms_key_self_link } whenever var.encryption is non-null, so the boot disk does get encrypted here. (The encrypt_boot field on that variable is declared but never read anywhere in the module, so it doesn't gate this.)

No new variables and no change to the key, keyring or IAM — the grants that were already there are what makes this work.