NEC plugin: PFC packet fitler support

NEC plugin: PFC packet fitler support

It also enhances the following points of packet filter code:
- Allow to clear filter fields to wildcard by specifying None
  in PUT method
- Return None for wildcard fields in an API response
- Determine eth_type based on protocol field on DB layer
- Support OFC driver-specific API validation and update support
- Refactor packet filter code and better validations

Implements blueprint nec-packet-filter-pfc
Change-Id: I2b208c66a9f119ef6d2ef78079d9a7e5965202e7