runtime recipe_module: is_experimental should default to False.
runtime recipe_module: is_experimental should default to False.

R=40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org

Bug:
Change-Id: Ib411812e8fc71b535d05e3ae65193efad6c3cbaf
Reviewed-on: https://chromium-review.googlesource.com/773624
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

luci/recipes-py,luci/recipes-py