Adds the `ImportStateIdPrefix` field for import acceptance tests. There are (albeit fairly rare) import cases where a resource needs to be imported with a combination of the resource's ID and a known string prefix. This allows the developer to specify the known prefix, and omit the `ImportStateId` field.
```
$ make test TEST=./helper/resource TESTARGS="-run=TestTest_importStateIdPrefix"
==> Checking that code complies with gofmt requirements...
==> Checking AWS provider for unchecked errors...
==> NOTE: at this time we only look for uncheck errors in the AWS package
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/30 18:08:36 Generated command/internal_plugin_list.go
go test -i ./helper/resource || exit 1
echo ./helper/resource | \
xargs -t -n4 go test -run=TestTest_importStateIdPrefix -timeout=60s -parallel=4
go test -run=TestTest_importStateIdPrefix -timeout=60s -parallel=4 ./helper/resource
ok github.com/hashicorp/terraform/helper/resource 0.025s
```
