vrouter-agent ignores Router Solicitations in L3-only mode

[Ankudinov Alexander]

When VM wants to autoconfigure ipv6 address it sends Router Solicitation to make routers answer immediately with Router Advertisement (see https://tools.ietf.org/html/rfc4861).

# Expected behaviour
Made an overlay ipv6 L3-only network with a virtual machine attached to it.

Sent Router Solicitation with rdisc6 and immediatly got an Router Advertisement in answer to Router Solicitation.

# Actual behaviour
No answer to Router Solicitation.

In contrail-vrouter-agent http interface(http://localhost:8085/Snh_SandeshTraceRequest?x=Packet) there is a message: "PktErr: bridging not enabled for interface index <5> controller/src/vnsw/agent/pkt/pkt_handler.cc 196"