Add Google Cloud Engine (GCE) provider

 * force kpartx to use synchronous mode, removing sleep(10)
 * get image configuration, use it during tarball creation and registration
 * add (non-working, path problems) image registration
 * add cleaning of image from Google keys and repositories
 * add NTP server address in manifest
 * add preference for backport kernels in manifest
 * disable IPv6
 * correctly set host name