Updates the aws_elb acceptance tests to better handle parallel test runs
```
$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSLoadBalancerPolicy_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/02/09 10:04:58 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSLoadBalancerPolicy_ -timeout 120m
=== RUN TestAccAWSLoadBalancerPolicy_basic
--- PASS: TestAccAWSLoadBalancerPolicy_basic (24.50s)
=== RUN TestAccAWSLoadBalancerPolicy_updateWhileAssigned
--- PASS: TestAccAWSLoadBalancerPolicy_updateWhileAssigned (42.34s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 66.866s
```