Add page.AMIPage, a page with AMI details for an event
    
    This is similar to page.DownloadPage, but includes details of
    the available EC2 AMIs for an event, retrieved from datagrepper.
    relval will be enhanced to produce this page, and the wiki
    test instructions template will be enhanced to show it on Cloud
    validation pages.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>