libvirt: pass Host object into firewall class

Instead of passing in a get_connection callback function,
give the firewall class impls direct access to the new Host
object instead. Make use of the fakelibvirt fixture to
avoid needing to mock out the libvirt APIs directly.

Blueprint: libvirt-driver-class-refactor
Change-Id: I8af9a5a2ba4f87ef221b9276df7d08395e44f021
Cleanup in test_availability_zone not to use wsgi_app

In API unit testing, making call through WSGI is little bit overhead,
wherever applicable, unit tests can make direct call to controller
methods.
Some unit tests need WSGI call to tests more than one plugin together.

This patch makes above cleanup in test_availability_zone.

Partially implements blueprint v2-on-v3-api

Change-Id: I4e5651631f5c9a3c6015ab603789fb155848448c
Merge "Cleanup in test_availability_zone not to use wsgi_app"
Merge "libvirt: pass Host object into firewall class"