Add option to specify --fixed-model-name

[rgstephens]

I'd like to be able to specify the model file name that is used after a successful train. I have a CI-CD pipeline and I want to push the trained model to an S3/GCP/Minio bucket. I need to know the name of the model file so it can be used in later CI-CD steps and when I pull the model at runtime with the --remote-storage option.