[lldpmgrd] To change error log condition in is_port_up()

[chenkelly]

- Why I did it
For lldpmgrd, listen for changes to the PORT table in the CONFIG_DB and APP_DB
To handle alias/description config change.
To check if port is up or down by looking into the oper-status for the port in PORT TABLE in the App DB.
Don't find it in the App DB, will log error.
During initializing, it is possible that port change in CONFIG_DB and not ready in APP_DB.
When we executed test_mgmtvrf.py::TestReboot::test_reboot case, it cannot pass due to LogAnalyzerError.

Apr 29 03:28:18.447385 as7726-32x-1 INFO lldp#lldpmgrd: Starting up...
Apr 29 03:28:18.451707 as7726-32x-1 ERR lldp#lldpmgrd: Port 'Ethernet8' not found in PORT_TABLE table in App DBApr 29 03:28:18.682655 as7726-32x-1 INFO swss#supervisord: start.sh portmgrd: started
.....
Apr 29 03:28:18.772995 as7726-32x-1 NOTICE swss#orchagent: :- initPort: Initialized port Ethernet8
Apr 29 03:28:18.953485 as7726-32x-1 NOTICE swss#portsyncd: :- main: PortInitDone
Apr 29 03:28:18.957141 as7726-32x-1 NOTICE swss#orchagent: :- doPortTask: Set port Ethernet8 admin status to up

- How I did it
To change error log condition, only in "port initialization procedure is done , but the port entry not found".

- How to verify it
To run test_mgmtvrf.py::TestReboot::test_reboot case, don't see the ERR log after DUT rebooting

Signed-off-by: kelly_chen@edge-core.com