Enable user to specify existing security groups

[jebbens]

A customer needs to specify preexisting security groups created by their agency's cloud hosting division - because they do not have permissions to create their own SGs - 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!