Error with multiple networks, when kwargs["nics"] is empty

Description
===========
There is a situation with several scenarios, where parameter 'auto_assign_nic' is not available (for randomly chosen network prepared by context) and also net-id was not set.

Steps to reproduce
==================
For example this scenario:
- title: NovaServers.boot_lock_unlock_and_delete tests
    scenario:
      NovaServers.boot_lock_unlock_and_delete:
        flavor:
          name: {{flavor_name}}
        image:
          name: {{image_name}}
    runner:
      constant:
        times: 1
        concurrency: 1
    contexts:
      network: {}

Actual result
=============
I want to use network created by context, but method (in rally_openstack/scenarios/nova/utils.py, NR 80) doesn't allow that:

if auto_assign_nic and not kwargs.get("nics", False):
    nic = self._pick_random_nic()
    if nic:
        kwargs["nics"] = nic

Because of not selected network, there is error with multiple network raised.

Logs & Configs
==============
novaclient.exceptions.Conflict: Multiple possible networks found, use a Network ID to be more specific. (HTTP 409)