Allow messages inside messages

Bundle Add Message and Request Forward Message, which is part of OpenFlow
1.4, encapsulates an OpenFlow message in side an OpenFlow message.

This patch prepares for this by adding a MsgInMsgBase class, a subclass of
MsgBase which allows its subclasses to include subclasses of either
MsgInMsgBase or MsgBase when when parsing classes from JSON: The MsgBase
class does not allow this.

This change has three parts:
* Remove the assertion in
  ofproto_parser.py:StringifyMixin::cls_from_jsondict_key()
  that cls is not a subclass of MsgBase.

* Pass **additional_args to various stringify.py:StringifyMixin decoder
  methods to make the datapath available when instantiating
  MsgBase subclasses.

* Override _decode_value() in MsgInMsgBase to pass **additional_args
  to decoder. The method in the parent class, StringifyMixin,
  does not pass **additional_args.

  The effect is to pass a datapath argument if
  the class is a subclass of MsgInMsgBase but not if the
  class is a direct subclass of MsgBase.

  By only making messages which allow messages inside them
  subclasses of MsgInMsgBase this allows the datapath
  argument to be passed to the decoder if and only if needed.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
of14: Add bundle add message support

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
of14: Add bundle add message unit test

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Add request forward message support

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Add request forward message unit test

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>