Duplicate RA messages are sending from ODL Controller

Duplicate RA messages are sending from ODL Controller

Issue:
======
While sending IPv6 periodic RA packets to all Ipv6
VMs OF port ODL controller is sending more than one
time with same RA packets to same ELAN group as a
result each IPv6 VMs OF ports are getting more
than one RA packets for each perodic
UNSOLICTED/CEASE messages.

Solution:
=========
Using IPv6 network ELAN BC group approach sending
IPv6 RA packet from ODL Controller to VMs OF ports
should be one time is sufficient.
Since ELAN BC group will deliver the RA packets to
all IPv6 VMs OF ports for that particular ELAN group.

Change-Id: Ia1564148ce84044754e6a008e1de21a62435459d
Signed-off-by: Nishchya Gupta <nishchyag@altencalsoftlabs.com>