Use ExternalId for all assume role calls

[awslabs]

Use ExternalId for all assume role calls

[bundyfx]

On top of this, the Role that is used by ADF to perform the initial base stack creation or update should have only the minimal required permissions available to it to create the base stack. If a user wants to add more resource into the base stack then the permissions should be extended to allow this. Otherwise we should default to least privilege access of the cross account access role.

[bundyfx]

For now, we're not going to add externalId for assume role calls, it doesnt add anything major in terms for security in this sense. Ideally we would have a way to lockdown the ADF cross account role IAM policy after it has setup the base stack. This could be a flag added to the adfconfig.yml to help secure everything down, but for now this is not mandatory.