Fix bug when PORT key is missing on flex counter table

[AharonMalkin]

After the following Change: #723
On some cases, when trying to override the counterpoll interval, the user gets a key error:

importlib._bootstrap>\\", line 621, in _exec\\n  File \\"<frozen importlib._bootstrap_external>\\", line 940, in 
exec_module\\n  File \\"<frozen importlib._bootstrap>\\", line 241, in _call_with_frames_removed\\n  File 
\\"/tmp/ansible_generate_golden_config_db_payload_9x6dtuml/__main__.py\\", line 341, in <module>\\n  File 
\\"/tmp/ansible_generate_golden_config_db_payload_9x6dtuml/__main__.py\\", line 337, in main\\n  File 
\\"/tmp/ansible_generate_golden_config_db_payload_9x6dtuml/__main__.py\\", line 324, in generate\\n  File 
\\"/tmp/ansible_generate_golden_config_db_payload_9x6dtuml/__main__.py\\", line 135, in 
generate_full_lossy_golden_config_db\\nKeyError: 'PORT'\\n",

To fix it, I added a setdefault method to create the key if it doesnt exist.

[sdszhang]

lgtm