backend: Disable compression in export info file

The new import/export infrastructure in Ganeti 2.2 and up handles
compression differently. It no longer writes compressed files to the
destination. Unfortunately changing this behaviour would be non-trivial,
so in the meantime setting “compression = none” will hopefully avoid
some confusion.

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