aws user should be configurable

[mattd-mb]

Currently the non-root user for provisioning defaults to ec2-user. This should be configurable.

https://github.com/shinesolutions/aem-aws-stack-provisioner/blob/master/manifests/pre-common.pp#L77

[mbloch1986]

It's already configurable from aem-aws-stack-provisioner point of view. But we need to allow the configuration of an aws user in the aem-aws-stack-builder configurations. and pass it down to pre_common::user