aws_alb_target_group will ForceNew resource
Fixes#8741
The modify-target-group doesn't allow changes to name, port, protocol or
vpc_id - therefore, they should all be ForceNew: true
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSALBTargetGroup_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/09/22 16:04:29 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSALBTargetGroup_ -timeout 120m
=== RUN TestAccAWSALBTargetGroup_basic
--- PASS: TestAccAWSALBTargetGroup_basic (50.66s)
=== RUN TestAccAWSALBTargetGroup_changeNameForceNew
--- PASS: TestAccAWSALBTargetGroup_changeNameForceNew (84.48s)
=== RUN TestAccAWSALBTargetGroup_changeProtocolForceNew
--- PASS: TestAccAWSALBTargetGroup_changeProtocolForceNew (95.89s)
=== RUN TestAccAWSALBTargetGroup_changePortForceNew
--- PASS: TestAccAWSALBTargetGroup_changePortForceNew (85.77s)
=== RUN TestAccAWSALBTargetGroup_changeVpcForceNew
--- PASS: TestAccAWSALBTargetGroup_changeVpcForceNew (85.00s)
=== RUN TestAccAWSALBTargetGroup_tags
--- PASS: TestAccAWSALBTargetGroup_tags (88.11s)
=== RUN TestAccAWSALBTargetGroup_updateHealthCheck
--- PASS: TestAccAWSALBTargetGroup_updateHealthCheck (82.15s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 572.083s
```
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