ofproto: change the way __str__ is implemented for of-wire messages

instead of explicit listing of of-wire attributes, use a heuristics
to exclude internal attributes. (eg. buf, datapath, etc)

this commit changes __str__ outputs.  update a test case accordingly.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>