Prevent dropping ARP packets

Prevent dropping ARP packets

JIRA: SAMPLEVNF-152

When system is overloaded, ARP packets were sometimes dropped, as any
other packets.
This was causing two issues:
- The count of TX non dataplane packets was wrong
- If many consecutive ARP packets were dropped, the underlying switch might
  see its ARP timer expiring, causing performance degradation (packets being
  broadcasted).
ARP packets are now always sent as no-drop.

Change-Id: I9a86cbf8c4b56a178f86bc789153f1fa49ddf73f
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>