python/samba/netcmd/schema.py: add schema show_oc for attribute

python/samba/netcmd/schema.py: add schema show_oc for attribute

Often administrators need to add a specific attribute to an object, but
it may not be possible with the objectClasses present. This tool allows
searching "what objectclasses must or may?" take an attribute to help hint
to an administrator what objectclasses can be added to objects to achieve
the changes they want.

Signed-off-by: William Brown <william@blackhats.net.au>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>