Switching from format("%s-a", var.region) -> var.zone on bastion resource

[simonbutt]

Switching from format(%s-a, var.region) -> var.zone on the bastion terraform resource. We already have zone as a variable and the previous doesn't work for the europe-west1 or us-east1 regions (which don't contain a "-a" zone)

Signed-off-by: simonbutt [email protected]

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[simonbutt]

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