Customization of staging models, datasets, and other assets

[luszczek]

Currently, templating during staging of models, datasets, and other assets is done through single {}'s braces in JSON files. This is easy to understand but a non-Pythonic way. Using string {dataset.source} in JSON file looks for dataset.source (attribute source of object dataset according to Python's format) even though Python's official format() syntax should be for dataset["source"]. This requires specialized code in executors.py that is not a part of the standard Python library or provided consistently with a short and simple code. The other problem is no means of customization for environments for staging or running commands. Some behavior of many commands can only be changed through environment variables. Both of these problems can be fixed by switching to string.Template for both JSON and shell customization which is consistent with the Python standard library and various command line shells.