VPNaaS: Separate validation for Cisco impl

Moving validation logic from Cisco service driver, into a
validation subclass, so that it'll be applied during the
persistence phase (so validation failures can block the
persistence, instead of setting to error state).

This is part of the effort to separate out L3 validation
and builds upon the reference implementation changes being
reviewed in 102351.

Note: One unit test is commented out, due to a bug in
oslo.messaging 1.3.0. It will be uncommented later, once
1.4.0.0.a3 or newer is available for use.

Change-Id: I6040e6d8ab32b707bcf67a0882507b7ee2d4bebe
Partial-Implementation: blueprint l3-svcs-vendor-validation
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"
Merge "VPNaaS: Separate validation for Cisco impl"