Encapsulate some port properties in the PortContext

Bindings to host or status may need further encapsulation
to avoid exposing mechanism drivers to underlying DB model
details. This was particularly true in the case of the
l2pop mech driver.

As a result, some docstrings were reworded, and the newly
introduced properties used directly in the mech drivers.

Partially-implements: blueprint neutron-ovs-dvr
Supports blueprint: ml2-hierarchical-port-binding

Change-Id: If2a373ef04d19b164585fb4bde4fe6e0cfaf43be
Merge "Encapsulate some port properties in the PortContext"