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
```
This dep was meant as a stopgap for a patch in the stdlib of Go which I
wrote.
The patch was released in Go 1.7. We can now remove my fork and go back
to the stdlib
The template resources don't actually need to retain any state, so they
are good candidates to be data sources.
This includes a few tweaks to the acceptance tests -- now configured to
run as unit tests -- since it seems that they have been slightly broken
for a while now. In particular, the "update" cases are no longer tested
because updating is not a meaningful operation for a data source.
Jake Champlin