change: refactor logic in fw_utils and fill in docstrings

[laurenyu]

Issue #, if available:

Description of changes:
There was a lot of duplicated logic, as well as some convoluted logic around Python versions, so I'm hoping to simplify it (and go back to having all the framework version thresholds in one place).

I also removed some of the usage of "merged" as it was confusing to some. (it referred to the merging of the SageMaker and DLC images, but was vague enough that at first glance it could mean the merging of training/inference images or something else like that.)

Testing done:

• tox -e flake8,pylint,py36 tests/unit
• also manually pulled the MXNet 1.4.1 EI images to verify which versions existed (I was surprised that the 1.4.1 images are indeed DLC images for both Python 2 and 3)

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