provider/aws: Updating the aws_efs_mount_target dns_name

[stack72]

Fixes:#10902

AWS introduced a change to the Mount Target DNS Name to remove the
availability_zone from it -
https://aws.amazon.com/about-aws/whats-new/2016/12/simplified-mounting-of-amazon-efs-file-systems/

This was because there used to be a limit of 1 mount target per AZ -
this has been raised.

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSEFSMountTarget_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/01/04 10:45:35 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSEFSMountTarget_ -timeout 120m
=== RUN   TestAccAWSEFSMountTarget_importBasic
--- PASS: TestAccAWSEFSMountTarget_importBasic (236.19s)
=== RUN   TestAccAWSEFSMountTarget_basic
--- PASS: TestAccAWSEFSMountTarget_basic (445.52s)
=== RUN   TestAccAWSEFSMountTarget_disappears
--- PASS: TestAccAWSEFSMountTarget_disappears (228.31s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	910.044s

[catsby]

I'm thinking ( 🤔 ), should this region-only DNS Name be a separate computed attribute? Is the AZ specific one still valid, or is it deprecated? I'm thinking that anyone referencing efs_mount_target.mount.dns_name would see a downstream change because of this. Maybe that's unavoidable if the AZ one is deprecated 🐼

[catsby]

so, I re-read #10902 and it seems the AZ is just dropped on AWS side? If so, then this LGTM 👍

[stack72]

Thanks @catsby :) yes, they dropped it on their side

[tjwallace]

Is there any point of calling getAzFromSubnetId if the return value isn't used?

[tjwallace]

Could the dns_name attribute also be added to the aws_efs_file_system resource now, since the value of efs_mount_target.dns_name will be the same for the same file_system_id?

[ghost]

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