Why I did it
Currently SONiC use the 'isc-dhcp-relay' package to allow DHCP relay functionality on IPv4 networks only.
This will allow the IPv6 functionality along the IPv4 type.

How I did it
Edit supervisord template to start DHCPv6 instances when configured to do so on Config DB.
Align cfg unit test to the new change.
Add DHCPv6 relay minigraph parsing support and a suitable t0 topology xml file for UT.

How to verify it
Configure DHCPv6 agents as described on the feature HLD: sonic-net/SONiC#765
Test it with real client/server with IPv6 or use the dedicated automatic test: sonic-net/sonic-mgmt#3565
Signed-off-by: Shlomi Bitton <shlomibi@nvidia.com>

* Split docker-dhcp-relay.supervisord.conf.j2 template into several files for easier code maintenance

(cherry picked from commit 604becd)

…into kellyyeh-dhcp6