Add support to change interface model for system testing

[Andrew Woodward]

We need to be able to support changing interface models at a high level so that testing for issues like splinters can be done more rapidly.

[Andrew Woodward (xarses)]

https://github.com/Mirantis/devops/pull/25 for changes to devops

[Mike Scherbakov (mihgen)]

> unit testing
you mean system?

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/59871

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/59871
Committed: http://github.com/stackforge/fuel-main/commit/04a94f5bb4cc3041183eb4871abca775cc5db4c5
Submitter: Jenkins
Branch: master

commit 04a94f5bb4cc3041183eb4871abca775cc5db4c5
Author: Andrew Woodward <email address hidden>
Date: Tue Dec 3 15:20:26 2013 -0800

    Add support to change inteface model (driver)

    In order to help with driver testing, this change adds INTERFACE_MODEL
     to the fuelweb_test settings.py and allows for it to be overridden with OS
     environment var of the same name. A quick change of this will cause all
     instances to inherit the same model so it doesn't have to be changed by
     hand.

    Requires https://github.com/Mirantis/devops/pull/25

    Will work with default of virtio with out devops #25, but all other models
     will require patch or will raise an exception in devops.

    Closes-bug #1257513

    Change-Id: I778c197a271eda1016b25b630f75e54462b61fb1