s4/torture/gpo: Use existing code to 'run' external commands

s4/torture/gpo: Use existing code to 'run' external commands

Noticed when the smb.conf defined 'gpo update command' contained
the $PYTHON version then the exec_wait function failed to run the
command. Seems there is some issue with the arg handling. Also
there is already existing code (samba_runcmd_send) that works fine
in similar situation (e.g. when running dnsupdate etc.) so replaced
the homebrewed exec_wait functionality with the samba_runcmd util
function.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>