pyldb: add Dn.ldb accessor

pyldb: add Dn.ldb accessor

This, and the next commit, might help in debugging when you see a
traceback that ends like this:

  File "/data/samba/samba/bin/samba_upgradeprovision", line 664, in add_missing_object
      delta.dn = dn
  RuntimeError: DN is from the wrong LDB

in this case you could force a solution with something like:

 delta.dn = ldb.dn(delta.ldb, str(dn))

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