This resource (unlike the others in this provider) isn't stateful, so it
is a good candidate to be a data source.
The old resource form is preserved via the standard shim in helper/schema,
which will generate a deprecation warning but will still allow the
resource to be used.
As of this commit this provider has only logical resources that allow
the creation of private keys, self-signed certs and certificate requests.
These can be useful when creating other resources that use TLS
certificates, such as AWS Elastic Load Balancers.
Later it could grow to include support for real certificate provision from
CAs using the LetsEncrypt ACME protocol, once it is stable.
Martin Atkins