The child_params calculation doesn't differentiate parameters
provided via the properties vs those provided via the template
defaults (defined in the nested stack template), which leads to
confusing results when you look at the parameters for the nested
stack, as the defaults are exposed as parameters passed in
to the stack, when the parent didn't pass anything via properties.
Closes-Bug: #1498107
Change-Id: Ic58cdcd15db09cd8d8d3d52fadfb68aa5709e7d8
a7ec465031