downgradedatabase: installing script

downgradedatabase: installing script

Installing downgrade script so people don't need the source tree for it.

Exception added in usage test because running the script without arguments
is valid. (This avoids the need to knownfail it).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14059

Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>