test cases for import models

[mayoor]

* Added Test cases for import model
* Changed logic around setting deployment container default command line options and the default environment variables. This information will be extracted from the container_index.json. This will allow us for adding multiple inference containers, without having to change the code
Minor fixes -
    * Deployment requires only BASE_MODEL env variable
    * Changed the return type for register API to return DataScienceModel
    * Auto inferencing of service managed container type from the container URI