Add preview for ResourceGroup

Override ``child_template`` and ``child_params`` to allow previewing of
ResourceGroup as a group of resources.

Implements: blueprint preview-stack (partial)
Change-Id: I165e31dcc1be5eddf00caa4209b1ff027b129cc6
Add preview for InstanceGroup

Override ``child_template`` and ``child_params`` to allow previewing of
InstanceGroup as a group of resources.

Implements: blueprint preview-stack (partial)
Change-Id: I821ac72e11921964acd5d8f1dd33a302edf3aa8b
Merge "Add preview for ResourceGroup"
Merge "Add preview for InstanceGroup"