[202012] Replace VLAN table with DHCP_RELAY table for verifying ipv6 helpers and add unittests

[kellyyeh]

Cherry-pick #10654

Why I did it

dhcp6relay reads ipv6 helpers from DHCP_RELAY table. Docker supervisord should rely on DHCP_RELAY instead of VLAN table to determine if dhcpv6 relay needs to be started. Added "vlan missing ip helper" dhcp relay unittest

How I did it

Change docker supervisord to check for DHCP_RELAY table for ipv6 helpers
Added unittests

How to verify it

Run dhcp_relay docker, ensure ipv6 helper is present is only DHCP_RELAY table and not VLAN table, and dhcp6relay should start up

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[Blueve]

@kellyyeh do we need this in 202012?

[kellyyeh]

@Blueve Yes this change

@kellyyeh do we need this in 202012?

Yes, this PR is for 202012. It already exists in master

sonic-buildimage/dockers/docker-dhcp-relay/docker-dhcp-relay.supervisord.conf.j2

Line 51 in adffbd4

{% if DHCP_RELAY and vlan_name in DHCP_RELAY and DHCP_RELAY[vlan_name]['dhcpv6_servers']|length > 0 %}

[prsunny]

@kellyyeh , can you please add [202012] to title and also the PR thats merged to master?

[yxieca]

@kellyyeh 202205 is already having dhcp in submodule. Please advance submodule head instead.

[kellyyeh]

@yxieca This change is in docker-dhcp-relay and sonic-config-engine, no changes in sonic-dhcp-relay

[yxieca]

@kellyyeh 202205 is already having dhcp in submodule. Please advance submodule head instead.

@kellyyeh we don't cherry-pick from lower branch to higher branch. Please move the flag to the master branch PR.