gpo: Move the file parse function to gp_ext

gpo: Move the file parse function to gp_ext

A file will always be read from the sysvol the
same way, but the data will be read differently.
This patch moves the parse function to gp_ext,
and requires subclasses to implement the read()
function to interpret the data.

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