OVSDriver: remove IND_OVS_ACTION_CHECK_NW_TTL

This is no longer needed now that the pipeline receives the full OVS key,
including the TTL.
allow pipelines to set the metadata OXM in packet-in messages

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.
Merge into master from pull request #146:
allow pipelines to set the metadata OXM in packet-in messages (https://github.com/floodlight/ivs/pull/146)