domain.py: Make schemaupgrade option work regardless of config

domain.py: Make schemaupgrade option work regardless of config

Currently the 'samba-tool domain schemaupgrade' command will only work
if the Samba config has the non-default option 'dsdb:schema update
allowed = yes'. The whole point of running this samba-tool option is to
upgrade the schema, so it would seem to make sense to bypass the setting
temporarily, in order to apply the schema updates successfully.

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