Add StackResource specific preview behavior

This allows previewing a StackResource not as a single resource, but as
resources within a stack.  In order to do that, resources that inherit
from StackResource must override both ``child_template`` and
``child_params`` methods to allow the stack to be parsed.  If any of
these methods is missing, the preview will default to being just the
StackResource itself, and will be returned as a single resource.

Implements: blueprint preview-stack (partial)
Change-Id: I03be7e58a6809f5061546a469b69165d6ca70a30
Add preview for NestedStack

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

Implements: blueprint preview-stack (partial)
Change-Id: I3acde4ccc31731aee1209c747fdbcc4446be63bc
Merge "Add StackResource specific preview behavior"
Merge "Add preview for NestedStack"