gpo: Dynamically load gp_exts

gpo: Dynamically load gp_exts

This loads Group Policy Client Side Extensions
similar to the way that they are loaded on a
Windows client. Extensions are installed to a
configuration file in the samba cache path where
they receive a unique GUID matched with the path
to the python gp_ext file. Classes which inherit
from the gp_ext class (as defined in gpclass.py)
will be dynamically loaded.

Signed-off-by: David Mulder <dmulder@suse.com>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>