Add aarch64 support to beaker.

- Allow distro import to recognise Red Hat Server distro as a RHEL distro
- write out aarch64/grubefi entries.  Unfortantely its different enough from efigrub.
  - need to pass machine specific dtb file, but this will go away RSN.
  - each host needs a filename entry in dhcpd.conf.  grub/efi won't fall through once grub.efi is
    loaded from tftpboot.  The only way around it seems to remove the entry.
- Update kickstart.py to put %end for new osmajor

Change-Id: Iaa16e4eb725e63317987dbf2c1d85daa7f823b5e