- Why I did it
Make DHCP relay docker an extension. DHCP relay now carries dhcp relay commands CLI plugin and has a complete manifest.
It is installed as extension if INCLUDE_DHCP_REALY is set to y.

DEPENDS on #5939

- How I did it
Modify DHCP relay docker makefile and dockerfile. Make changes to sonic_debian_extension.j2 to install sonic packages.
I moved DHCP related CLI tests from sonic-utilities to DHCP relay docker.
This PR introduces a way to write a plugin as part of docker image and run the tests from cli-plugin-tests directory under docker directory.
The test result is available in target/docker-dhcp-relay.gz.log:

[ REASON ] :      target/docker-dhcp-relay.gz does not exist   NON-EXISTENT PREREQUISITES: docker-start target/docker-config-engine-buster.gz-load target/python-wheels/sonic_utilities-1.2-py3-none-any.whl-in
stall target/debs/buster/python3-swsscommon_1.0.0_amd64.deb-install
[ FLAGS  FILE    ] : []
[ FLAGS  DEPENDS ] : []
[ FLAGS  DIFF    ] : []
============================= test session starts ==============================
platform linux -- Python 3.7.3, pytest-3.10.1, py-1.7.0, pluggy-0.8.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /sonic/dockers/docker-dhcp-relay/cli-plugin-tests, inifile:
plugins: cov-2.6.0
collecting ... collected 10 items

test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_plugin_registration PASSED [ 10%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_add_dhcp_relay_with_nonexist_vlanid PASSED [ 20%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_add_dhcp_relay_with_invalid_vlanid PASSED [ 30%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_add_dhcp_relay_with_invalid_ip PASSED [ 40%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_add_dhcp_relay_with_exist_ip PASSED [ 50%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_add_del_dhcp_relay_dest PASSED [ 60%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_remove_nonexist_dhcp_relay_dest PASSED [ 70%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_remove_dhcp_relay_dest_with_nonexist_vlanid PASSED [ 80%]
test_show_dhcp_relay.py::TestVlanDhcpRelay::test_plugin_registration PASSED [ 90%]
test_show_dhcp_relay.py::TestVlanDhcpRelay::test_dhcp_relay_column_output PASSED [100%]

=============================== warnings summary ===============================
/usr/local/lib/python3.7/dist-packages/tabulate.py:7
  /usr/local/lib/python3.7/dist-packages/tabulate.py:7: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    from collections import namedtuple, Iterable

-- Docs: https://docs.pytest.org/en/latest/warnings.html
==================== 10 passed, 1 warnings in 0.35 seconds =====================