Tracing enhancements.

Tracing enhancements.

Limit buffer tracing to 50 in order to limit large output, unless
the user over rides the max "sh trace max <number>".

Add trace filtering, to be able to only trace packets that were
processed by a specific node or exclude packets processed by a node.
Example, only include packets processed by error-drop:
  # trace filter include error-drop 1
  # trace add dpdk-input 1000000
  <wait for packets, to come in>
  # show trace

Change-Id: I5d9e15d2268ea55e6ef87b2b8756049c49b2791b
Signed-off-by: Todd Foggoa <tfoggoa@cisco.com>