[dhcp-relay]: Launch DHCP Relay On L3 Vlan

[tahmed-dev]

Recent changes brought L2 vlan concept which do not have DHCP
clients behind them and so DHCP relay is not required. Also,
dhcpmon fails to launch on those vlans as their interfaces
lack IP addresses. This PR limit launch of both DHCP relay
and dhcpmon to L3 vlans only.

singed-off-by: Tamer Ahmed tamer.ahmed@microsoft.com

- Why I did it
Stop non functioning DHCP relay from being launched and also disable error being thrown by dhcpmon

- How I did it
Added extra check for presence of vlan in the vlan_interface table

- How to verify it
PR check passes

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

• 201811
• 201911
• 202006
• 202012

[abdosi]

we don't need this for 201811 ?

[prsunny]

lgtm.

[tahmed-dev]

we don't need this for 201811 ?

No, L2 Vlans are not 201811

[lguohan]

where is the expected results?

[tahmed-dev]

where is the expected results?

It is still one L3 vlan, however new L2 vlan does not have dhcprelay launched. It is a -ve test where without the change in the supervisord, the test fails with the following logs:

======================================================================
FAIL: test_dhcp_relay (tests.test_j2files.TestJ2Files)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/sonic/src/sonic-config-engine/tests/test_j2files.py", line 72, in test_dhcp_relay
    self.assertTrue(filecmp.cmp(os.path.join(self.test_dir, 'sample_output', utils.PYvX_DIR, 'docker-dhcp-relay.supervisord.conf'), self.output_file))
AssertionError: False is not true

----------------------------------------------------------------------
Ran 146 tests in 40.672s

FAILED (failures=1)