Fixes an uncaught panic during an interface cast in the `template_cloudinit_config` data source.
Fixes: #13572
```
$ make test TEST=./builtin/providers/template TESTARGS="-v -run=TestRender_handlePanic"
==> 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/)
2017/04/12 10:46:33 Generated command/internal_plugin_list.go
go test -i ./builtin/providers/template || exit 1
echo ./builtin/providers/template | \
xargs -t -n4 go test -v -run=TestRender_handlePanic -timeout=60s -parallel=4
go test -v -run=TestRender_handlePanic -timeout=60s -parallel=4 ./builtin/providers/template
=== RUN TestRender_handlePanic
--- PASS: TestRender_handlePanic (0.00s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/template 0.028s
```
53fe7c2293