Bug 1254 - OFP test coverage increase

- PacketInUtil is final with private constructor instead of being abstract
- added unit test for PacketInUtil#getMdSalPacketInReason method

Change-Id: I260c3f92580187de93b4ce7a16512c4a13b9882b
Signed-off-by: Martin Bobak <mbobak@cisco.com>
Merge "Bug 1254 - OFP test coverage increase"