Don't inherit from NestedStack

The NestedStack class is intended only to be the implementation of the
AWS::CloudFormation::Stack resource type. Other resources whose internal
implementation uses a nested stack should subclass the abstract
StackResource class.

Change-Id: Ic311c220dbf9bdf539d68f4004c8dc14752e057e
Signed-off-by: Zane Bitter <zbitter@redhat.com>