StackSets Takes StackTags and Retry Strategy When Throttling

[xiwhuang]

Issue #, if available:

Description of changes:

• StackSet handlers take stack tags
• Add retry strategy for CloudFormation client in StackSet

Testing

• Manual create/update stack with stack tags.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[xiwhuang]

Tested to create 20 StackSets with 20 StackInstances at same time.