EmrEtlRunner: Allow ec2_key_name to be undefined in config

[istreeter]

The EmrEtlRunner rejects the config if ec2_key_name is not defined, even though the job can run perfectly fine without a key pair.

We have use cases where it is important to run EER without allowing ssh capabilities to EC2. We can successfully hack around this problem by providing the empty string as the the key name. It would be much better to make it an optional parameter.