python/samba/netcmd/{forest.py,main.py}: add configuration controls

python/samba/netcmd/{forest.py,main.py}: add configuration controls

With samba-tool we should expose ways to easily administer and control
common configuration options. This adds the base framework for modifying
forest settings, generally stored in cn=configuration partition.

An example is:

samba-tool forest directory_service show
samba-tool forest directory_service dsheuristics X

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