ImportExport: use max and min memory params

Import uses the old "memory" parameter to populate the two new ones, if
they're not overridden already.

FinalizeExport exports minmem and maxmem, but also memory, as maxmem, to
allow importing to older ganeti clusters.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>