Allow router-gateway-set to work even without an assigned
subnet with the net_id so as to enable IPv6 L3 routing
using the assigned LLA for the gateway.
The goal is to allow for IPv6 routing using just
the allocated LLA address for the gateway port to be
used as the external gateway to connect to the upstream
router. For this purpose router-gateway-set no
longer has a requirement of an assigned subnet.
A new config has also been added to the l3_agent.ini
to allow the user to set a valid ipv6_gateway address
to be used as the gateway for the default ::/0 route
If the ipv6_gateway config is not set and a gateway
is still created without a subnet, the gateway interface
will be configured to accept router advertisements (RAs)
from the upstream router so as to build the default route.
Unit test changes and additions reflect these changes.