Make YouTube mocked server serve YouTube API

1.) Making YouTube mock server serve YouTube API.
2.) Adding ability to block YouTube API from YouTube mock server.
3.) Adding acceptance tests that check YouTube mock server for availability of YouTube API, and for it's ability to block YouTube API.

Part of BLD-939.
Merge pull request #3085 from edx/valera/make_youtube_mocked_server_serve_youtube_api

Make YouTube mocked server serve YouTube API