Allow use of default key for ssm parameters
Allow use of default key for ssm parameters

The last change to fleece made it possible to encrypt the rendered
SSM parameters using a custom KMS key, but in the process broke the
ability to use the default key.

That's because boto3's ssm put_parameter call is sort of confusing;
the way to use the default is to avoid passing in the `KeyId` arg
entirely, but the code was setting it to None which led to a runtime
error.

racker/fleece,racker/fleece