Sort OS names and variants in LUDiagnoseOS

The OS list and variants as returned from LUDiagnoseOS is not sorted,
and gnt-instance reinstall doesn't sort it either. This means that it
the menu that users are present with is inconsistent across clusters,
and that is confusing.

To make this consistent across all users of the LU, we sort the names in
the LU itself.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>