Implement sysName OID

[qiluo-msft]

Description

Steps to reproduce the issue:

1. Make sure all the containers running well and snmp is responding query on sysName OID (1.3.6.1.2.1.1.5.0)
2. Change the hostname in the SONiC host
3. Verify the hostname inside snmp container are still the old hostname

Describe the results you received:
Query sysName OID and the response is still the old hostname

Describe the results you expected:
Query sysName OID and the response should be the new hostname

Additional information you deem important (e.g. issue happens only occasionally):
Currently the OID is served by snmpd default implementation, so it fetch the hostname from inside snmp container. Suggest override the implementation in sonic-snmpagent repo. The ground truth of hostname should be from ConfigDB, as below command line

redis-cli -n 4 hget "DEVICE_METADATA|localhost" "hostname"

Output of show version:

Applicable to any version

Attach debug file sudo generate_dump:

Not related