Enable cross account lookup for AMIs

[gbvanrenswoude]

Enable crossaccount lookup when passing in the ROLE_ID and ACCOUNT_ID variable. When not passing them, use it as default.

[itsdalmo]

I just left another PR to support aws_session_token, saw this one and figured I'd leave my 2c:

Personally, I don't think resources should encourage users to add privileges to the workers instance profiles, since this means that all teams/pipelines/tasks will have the same privileges. Besides, I think this gives the AMI resource too many responsibilities.