[RFE] [IPv6] ping6 support for router interface on a _multi_ network

Currently, tempest has 8 Scenario tests that validate some IPv6 use-cases (like IPAM) along with ping6 to the neutron router-interface.

We have support for ping6 for a dual-stack network as well as IPv6 only network. However, multiprefix networks (i.e., a network with multiple IPv6 subnets) has some gaps and is not officially supported in Netvirt.

The following 4 tempest tests fall in the first category (i.e., dual-stack/single IPv6 network use-case) and they are supported. These 4 tests are validated in upstream OpenDaylight CSIT today.

test_dualnet_slaac_from_os
test_dualnet_dhcp6_stateless_from_os
test_dhcp6_stateless_from_os
test_slaac_from_os

The next set of 4 tempest tests use multiple IPv6 subnets in a network and they are not supported. This bug is to track the progress of this use-case.

test_dualnet_multi_prefix_dhcpv6_stateless
test_dualnet_multi_prefix_slaac
test_multi_prefix_slaac
test_multi_prefix_dhcpv6_stateless