VPP-117: Fix ip4 and ip6 lookup and rewrite traces

VPP-117: Fix ip4 and ip6 lookup and rewrite traces

Previous patch was printing wrong data since rewrite node traces
an ethernet packet while other ip nodes trace an IP packet.

This patch introduces different tracing functions for:
- lookup node (where the fib index is valid)
- rewrite node (where an ethernet frame is available)
- other ip nodes (only the ip header is traced)

Change-Id: I5971a2e89ae8668f5aed4a410565a5f27e01fc22
Signed-off-by: Pierre Pfister <ppfister@cisco.com>
(cherry picked from commit 584b99a0120778108bd019b697639fbf3c1505d1)