Stop sending gratuitous arp when ip version is 6

Stop sending gratuitous arp when ip version is 6

This fix prevents calls to the arping utility for IPv6
addresses, thereby eliminating errors reported by arping
for IPv6 addresses.

The assumption is that NDP, DAD, and RAs are sufficient
for address resolution and duplicate address detection
for IPv6, and that unsolicited Neighbor Advertisements (NAs)
are not required for OpenStack services. If this turns out
not to be the case for some service/feature, then a separate
bug should be filed to add support for unsolicited NAs for
that service.

Change-Id: I14f869b7d488d7e691f7316eafcab3064e12cda6
Closes-Bug: 1357068