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
Merge "Add support to change inteface model (driver)"