Do not assume networks should be assigned

There are several places within the Nova benchark scenarios where we
are assuming networks should be assigned. This breaks a variety of
different deployments. Instead of always assigning a network, do so
based on whether a new scenario argument of auto_assign_nic=True.

Change-Id: I4e2dd58629f3c07b9928a95879056a79382fcf8b
Closes-Bug: #1353622