Pull and parse parent devfile from registry in library

[yangcao77]

Currently devfile library can only parse parent devfile if uri is specified.

This issue is to support registry id defined under parent.
If RegistryUrl is defined, then pull from that specific registry. If not, then pull from default devfile registry.
The registry REST API should follow: https://github.com/devfile/api/blob/master/docs/proposals/registry/index-server-REST-API.md, the devfile can be got via GET request to GET http://{registry host}/devfiles/{stack}

The parent devfile should be parsed & override & merge into the flattened devfile.