samba-tool: Add option --keep-stale-entries to "samba-tool domain exportkeytab"

samba-tool: Add option --keep-stale-entries to "samba-tool domain exportkeytab"

This will keep stale keys in the keytab, which may be useful for wireshark
but is not correct if the keytab is used for accepting Kerberos tickets,
as tickets encrypted with old passwords would still be accepted.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jo Sutton <josutton@catalyst.net.nz>