cloud: set Google root size to 10 GB and EC2 back to 5 GB
    
    It's the *Google* image that's required to have a 10 GB root
    for performance reasons, not the EC2 image, as the comment says,
    but the change was inadvertently applied to the EC2 image not
    the Google one. This means our Google image is slow and our EC2
    images are failing to be published as AMIs.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>