Enable user to specify IAM roles

[jebbens]

A customer needs to specify preexisting IAM roles - because they do not have permissions to create IAM and application resources with the same roles - so that they can deploy LISA into their managed account.

[estohlmann]

Hey @jebbens, thank you for bringing this to our attention! We will review this PFR for potential inclusion in our next release. I will keep you up to date here!