Moved stub servers to terrain

Refactored stub services for style and DRY

Added unit tests for stub implementations

Updated acceptance tests that depend on stubs.

Updated Studio acceptance tests to use YouTube stub server; fixed failing tests in devstack.

common/djangoapps/terrain/__init__.py

@@ -4,3 +4,4 @@
 from terrain.browser import *
 from terrain.steps import *
 from terrain.factories import *
+from terrain.start_stubs import *