Add list-networks test for os-networks API

Adds a test to list networks for a non-admin tenant
using the compute os-networks API.

For Flat* network managers in nova, a tenant is not
assigned to a network so this tests that when listing
networks with a non-admin tenant, we get something back
since the underlying code in nova should pass 'allow_none'
to the DB API when listing the networks for Flat and
FlatDHCP network managers.

Change-Id: I5c50607279a8a385ebe96bf498e3e95079d6dec1
Related-Bug: #1327406
Merge "Add list-networks test for os-networks API"