allow pipelines to set the metadata OXM in packet-in messages

Reviewer: @poolakiran

Only 56 bits of metadata are supported because the packet-in reason takes 8
bits and the userdata nlattr (in old kernels) is only 64 bits.

This doesn't implement the full metadata matching and setting defined by OF
1.3. That will be possible after I do some refactoring of the action
translation code.