DOCS 0.5: Bug fix for getting started brooklyn.properties.

Added `user = ubuntu` for all getting started AWS locations (using Ubuntu 12.04 LTS AMIs).

Was experiencing `Repeater org.jclouds.rest.AuthorizationException` and `Unable to extract IP for <instance>` when running web cluster demo on AWS, from AWS hosted management node.

User was being passed as `ec2-user` from management node (AWS Linux), but Ubuntu AMIs require `user = ubuntu`. (Curiously, I don't remember this problem arising when using my local machine as the management node. )