Update AWSCloudFormationStackSetExecutionRole to include itself as principal

[silavjy]

AWS has changed its policy regarding trust policies on roles granting permission to principals.

"...a role trust policy must explicitly grant permission to the principals, including the role itself, that need to assume it under the specified conditions..We are contacting you because our data suggests that your AWS account may have one or more IAM Roles that assumes itself based on the permissions and conditions in its identity-based policy without explicitly granted permission in its role trust policy."

More to be found on https://aws.amazon.com/blogs/security/announcing-an-update-to-iam-role-trust-policy-behavior/