ML2 mechanism driver access to binding details

The following properties are added to the PortContext object passed to
ML2 mechanism drivers for port operations:

* bound_driver - name of current bound driver
* original_bound_driver - name previously bound driver in an update
* original_bound_segment - network segment used in previous binding

Some issues with the existing ML2 port binding unit tests were also
fixed.

The remainder of the fix for bug 1276395, making these binding details
available to mechanism drivers when a port is deleted, will be
addressed as part of the fix for bug 1276391.

Partial-Bug: #1276395
Change-Id: I9ecff4a4e044920ed2dde709c89aeb9bc773220d
Merge "ML2 mechanism driver access to binding details"