Allow multiple AWS account credential sources

[jgreenemi]

Customize the method by which the AWS keys are decided. User may want to create their training stack under a different AWS account than their environment variables are set, and the tool right now is set to always launch in the default creds' account.

Can introduce as part of a config file to set the credential retrieval method, and the ACCESS_KEY to use. Make sure not to ask that the SECRET_KEY be loaded into a config file as this is not secure by default and we shouldn't encourage users to keep plaintext creds on their computers.