Make availability zones a computed attribute.
Make availability zones a computed attribute.

Fixes a diff calculation error when only a VPC zone
identifiers is provided. In this case the associated
availability zones are computed from the subnets per
the AWS documentation.

nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,ashinohara/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,ClementCunin/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,ClementCunin/terraform-provider-aws,kjmkznr/terraform-provider-aws,ashinohara/terraform-provider-aws,nbaztec/terraform-provider-aws,ClementCunin/terraform-provider-aws,ClementCunin/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws