If the request doesn't return all of the remaining results,
NextToken is set to a token. To retrieve the next set of results, call
ListStackInstances again and assign that token to the request object's
NextToken parameter. If the request returns all results,
NextToken is set to
Summaries => ArrayRef[Paws::CloudFormation::StackSetSummary]
A list of
StackSetSummary structures that contain information about the user's stack sets.