Use discovery for ML API client instead of static file

[cymarechal-devoteam]

Using CloudTuner previously led to erroneous URLs in API calls, e.g.

https://us-central1-ml.googleapis.com/v1/projects/my-project/locations/europe-west4/studies?studyId=MyStudyID&alt=json

This was due to the static discovery file in src/python/tensorflow_cloud/tuner/api/ml_public_google_rest_v1.json which statically points to https://us-central1-ml.googleapis.com.

This modification aims to remove the static dependency to us-central1 and instead use the dynamic version of Discovery.

Reference:

• Google Cloud - AI Platform - Regional endpoints