improvement: remove SA attached to GCE

[Shabirmean]

Fixes #424

Description

• The Issue Remove associating ServiceAccount to GCE VMs #424 description has the main details.
• We have to remove direct Service Account association to GCE VMs
• Alternatively, we copy Service Account keys into the GCE VM and use for the actions that need it
• To support creation of VMs without Service Accounts, we need a fix in the terraform-google-vm repository
• The fix was added in improvement: allow null service_account terraform-google-modules/terraform-google-vm#262
• We have wait for the next release of the terraform-google-vm repository for us to be able to use the fix
• The changes in this repo uses the next release version (7.9.0) of the instance_template module. Thus the builds will fail until the release is published ()

Change summary

• Remove association of Service Account in the instance template that is created
• Copy the Service Account used for running Terraform into the GCE VMs once they are created
• Update the scripts to make use of this Service Account when running their commands
• Fix broken tests

Related PRs/Issues

• chore(master): release 7.9.0 terraform-google-modules/terraform-google-vm#263
• improvement: allow null service_account terraform-google-modules/terraform-google-vm#262
• cleanup: remove service account in GCE VMs #385
• improvement: add var for service account for GCE VMs #120

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[minherz]

lgtm with a couple of nit remarks.