* Make IoT certificate id generation deterministic
Fixes#3321
As per https://stackoverflow.com/questions/55847788/how-does-aws-iot-generate-a-certificate-id,
the IoT certificate ID is the SHA256 fingerprint of the
certificate. Since moto doesn't generate full certificates we will
instead use the SHA256 hash of the passed certificate pem.
* Don't allow duplicate IoT certificates to be created
Fixes#3320
When using boto3, trying to register a certificate that already
exists will throw a ResourceAlreadyExistsException. Moto should
follow the same pattern to allow testing error handling code in
this area.
958e95cf5c