Support Docker V2 Registries

[suneeta-mall]

• Support registries such as dockerhub, GCR and others that uses Docker V2 Registry API.

• Also remove https://github.com/heroku/docker-registry-client library for dockerhub support as its no longer maintained and some of the API we need eg. PutManifestAPI has issues. Instead look at using https://github.com/docker/distribution/blob/master/registry/client/repository.go instead. API https://github.com/docker/distribution/blob/master/docs/spec/api.md for reference or any other better solution

Also requested in:

• issue 31