Fix multiple videos on page

If there are several videos on page, make sure that
the global onYouTubeIframeAPIReady function is properly
setup so that all callbacks are trigerred when the YouTube API
loads.

BLD-972
Adding acceptance test for multiple videos.

Part of BLD-972.
Merge pull request #3124 from edx/valera/fix_multiple_videos_on_page_RELEASE

Valera/fix multiple videos on page release