python/samba/netcmd/schema.py: add schema query and management.

python/samba/netcmd/schema.py: add schema query and management.

Schema management in active directory is complex and dangerous. Having
a tool that safely wraps administrative tasks as well as allowing query
of the schema will make this complex topic more accessible to administrators.

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