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
```
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