VPP-117: Add trace to ip4 and ip6 lookup nodes

VPP-117: Add trace to ip4 and ip6 lookup nodes

The absence of trace in ip lookup nodes is misleading to many people.
This patch adds ip lookup tracing and therefore contribute to
worldwide happiness.

In addition, this patch makes sure sw_if_index[VLIB_TX] is
considered when tracing the fib_index value. In ip4/6-rewrite,
the value corresponds to the tx interface index. The formatting
function is therefore modified to take that case into account.

Change-Id: I5915f0446a15c45e391eedfdfcedd9057aa6a237
Signed-off-by: Pierre Pfister <ppfister@cisco.com>