Handle port deletion while updating lldp

[vasant17]

- What I did
Handled port deletion in lldpmgrd.

- How I did it
During dynamic port breakoput, we delete and add a port. When we delete a port, the port may NOT be present in one or both DBs (CFG and APP).

The goal is to configure the lldp when the port is present in both DBs and it has at-least alias or description set and the port is in oper_status UP.

- How to verify it
By running DPB command and checking the syslog messages

Before fix following error messages were seen in syslog:

ERR lldp#lldpmgrd: Port 'Ethernet1' not found in PORT table in Config DB. Using port name instead of port alias.
Mar 9 20:51:35.299500 lnos-x1-a-asw01 ERR lldp#python: :- pops: Failed to get content for table key PORT_TABLE:Ethernet1
Mar 9 20:51:35.301245 lnos-x1-a-asw01 ERR lldp#python: :- pops: Failed to get content for table key PORT_TABLE:Ethernet3
Mar 9 20:51:58.429793 lnos-x1-a-asw01 ERR lldp#lldpmgrd: Port 'Ethernet1' not found in PORT_TABLE table in App DB
Mar 9 20:51:58.441543 lnos-x1-a-asw01 ERR lldp#lldpmgrd: Port 'Ethernet2' not found in PORT_TABLE table in App DB
Mar 9 20:51:58.447253 lnos-x1-a-asw01 ERR lldp#lldpmgrd: Port 'Ethernet3' not found in PORT_TABLE table in App DB

After fix the error messages were NOT seen.
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