oem-ec2-compat: Check for metadata service and config drive

Allow the existing media-configdrive.service unit to run cloudinit on
/media/configdrive. If there is no such mount, ec2-cloudinit-metadata.service
will run cloudinit on the metadata service instead. This will allow these
instances to work with both a configdrive or metadata service. Additionally,
get rid of the helper scripts since cloudinit can read from the metadata
service now.