Update client os lists to name+variant format

List of OSes are displayed by gnt-os list, rapi, and gnt-instance
reinstall --select-os, and checked by burnin. In all of these show the
list with name+variant, if the os has variants.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>