(gh-477) enables root for non-root users on ec2 hypervisor

- root login is not possible on ubuntu-amis
- provision is not possible without root

This Patch enables root when the host['user'] differs from "root"
Better to just test the method call to enable_root_login is tested here, not the exact commands since this is a unit test specific to aws_sdk. This kind of test belongs in a test for enable_root_login really.