kiji-model-repository: KIJIREPO-55. Update the model_container to support specifying training parameters

issue: https://jira.kiji.org/browse/KIJIREPO-55
review: https://review.kiji.org/r/1881
commit: https://github.com/kijiproject/kiji-model-repository/commit/e8ac5abf1d2c93fb09aaadccfd2c18b93cdf10b9