improvement: allow null service_account

[Shabirmean]

Description

• The service_account variable for the instant_template module is a required variable
• This variable is used as part of the content section of the dynamic block for the google_compute_instance_template resource from the provider
• Thus, it is not possible to create an instant template with no service account associated with it
• As a result it's not possible to go from the instant_template module in this repo to a GCE VM which has no service account associated with it

Why is it an issue

• This is friction full in certain use-cases; for example when installing Anthos bare metal on GCE VMs that have a Service Account associated with it; there are some parts of the install that conflict with this service account
• Thus we want to be able to create instance_templates which has no service account attached

[comment-bot-dev]

Thanks for the PR! 🚀
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[bharathkkb]

Thanks @Shabirmean