Will/fix video acceptance tests

This fixes failures in the videoalpha acceptance tests.

* Resolves a name conflict in Lettuce steps.  Previously, two steps matched "the course has a Video component": one for old-style videos and one for video-alpha.  The old-style video step would pass, but the video alpha step would fail.  This explains why the acceptance tests were able to pass in Jenkins but failed locally.

* Fixes the template name used to create VideoAlpha components.  The old name "default" was not the correct name of the template.

* Changes stubbing behavior so that the video player loads, but the video itself does not.  This allows the tests to more closely replicate the development/production environment.  It is also necessary for the video alpha tests to pass, since the tests look for a ".video" element.

Suggested reviewer: @valera-rozuvan and @jzoldak

@nedbat : Could you please confirm that this resolves the failures you were seeing in your local dev environment?