dpset: absorb *args, **kwargs passed to __init__

dpset might be initialied implicitly like OFPHandler.
In that case, _CONTEXTS are passed to dpset. so we should ignore them.

Signed-off-by: ISHIDA Wataru <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>