L2 BD API to flush all IP-MAC entries in the specified BD

Implement API/CLI to clear IP-MAC tables used for ARP-termination
in the specified bridge domain.
The CLI to flush MAC IP tables for a BD is:
 set bridge-domain arp entry <bd-id> del-all
The API added is bd_ip_mac_flush.

Change-Id: I34ceb87c0f480c7102f6559312c24081ed485af8
Signed-off-by: John Lo <loj@cisco.com>