source4/scripting/python/samba/samba3: handle ntdb files.

source4/scripting/python/samba/samba3: handle ntdb files.

Upgrading old Samba 3 instances seems like a place where we don't have
to read ntdb files, but Andrew Bartlett points out that you can run a
Samba 4.0 and even a 4.1 'classic' domain and desire to migrate that
to the AD DC.

So make this upgrade code generic: if it finds an ntdb file, read
that, otherwise read the tdb file.

Cc: Jelmer Vernooij <jelmer@samba.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Reviewed-by: Jeremy Allison <jra@samba.org>