Move skip tests list to a nosetest config file

[sandeep-krishnamurthy]

Currently, for MXNet backend, we skip around 190 tests out of ~650 tests. We skip tests with @skip_if decorator. This involves changes in the code. Pulling latest from Keras and merging into this repo, creates merge conflicts. It would very useful if we can remove those MXNet specific skip test code from test files and put it as a config or have a test runner that manages the list of tests to run or skip.

One closest reference is ONNX tests - https://github.com/onnx/onnx/blob/master/onnx/backend/test/runner/__init__.py