Improve packet-filter test coverage in NEC Plugin

Improve packet-filter test coverage in NEC Plugin

blueprint nec-plugin-test-coverage

This commit adds unit tests for packet-filter in NEC Plugin.

This commit refactors packet-filter in NEC Plugin.
- Put packet-filter classes and methods into nec/packet_filter.py (a) and
  nec/db/packetfilter.py (b), NEC Plugin specific codes are in (a)
- Change stateless methods to class methods in extenstions/packetfilter.py
- Add 'convert_to' option to the attribute map of packet-filter to convert
  some string parameter to int at the api layer

Also, this commit includes the following changes in packet-filter.
- Fix attribute map of packet-filter; set in_port to allow_put=False
- Add new methods to update attribute map properly
- Make packet-filters ignore status of associated resource (network)

Change-Id: I7c0b76afb603f1f078b28610181b16ce66225d37