sfputilbase.py to utilize portconfig.py instead of parsing config files directly

[jleveque]

sfputilbase.py should utilize sonic-config-engine/portconfig.py to parse port configuration files, rather than doing it directly.

Eventually, the functionality in portconfig.py should be moved out of sonic-config-engine and into sonic-py-common, which is tracked by https://github.com/Azure/sonic-platform-common/issues/110