Reduce verbose log about mgmt ports

[qiluo-msft]

Description

Steps to reproduce the issue:

1. Running the SONiC image on a M0 switch
2. Observe the /var/log/syslog file

Describe the results you received:

2020-10-24 12:54:22.6907595 <28>Oct 24 12:54:22 sonic_m0 snmp#snmp-subagent [sonic_ax_impl] WARNING: Invalid management IP
2020-10-24 12:54:22.6917967 <28>Oct 24 12:54:22 sonic_m0 snmp#snmp-subagent [sonic_ax_impl] WARNING: Invalid mgmt IP
2020-10-24 12:54:22.6919091 <28>Oct 24 12:54:22 sonic_m0 snmp#snmp-subagent [sonic_ax_impl] WARNING: Invalid management IP
2020-10-24 12:54:56.4343259 <28>Oct 24 12:54:56 sonic_m0 snmp#snmp-subagent [sonic_ax_impl] WARNING: No managment ports found in b'MGMT_PORT|'

Describe the results you expected:
I see multiple issues in the syslog.

1. M0 by design has no management ports locally, so the ConfigDB has no MGMT_PORT table. We don't need to warn on this
2. LLDP message from neighbor contains empty management port on their side. No need to warn on this

Additional information you deem important (e.g. issue happens only occasionally):

**Output of `show version`:**

Any 201811 image and future

**Attach debug file `sudo generate_dump`:**

See above snippet