MacAddress objects are not being provided in UnitTests

[MichaelWasher]

MacAddress objects are used to represent both the src_mac and port_id of packets in some functions. MacAddress objects should be used to represent mac values at all times, but due to Pythons use of duck-typing this has slipped through.

An example of this is here:
https://github.com/faucetsdn/chewie/blob/master/test/unit/test_chewie.py#L292-L299
https://github.com/faucetsdn/chewie/blob/master/test/unit/test_chewie_mocks.py#L47

The documentation of Chewie claims MacAddress is expected and is assumed below port_up() / port_down() functions.

Change the tests provide the correct object type and clean the use of mac address.
There should be no need for the use of str(port_id) as the __str__ function should be called internally in the log/print functions.