Set default_network in tempest.conf

This is to support testing of change
Ia78582cac3790653c2281a5b63d953cd46d5c290 in Tempest.

Change-Id: Ibb812e2598fb11b7eef21a0868ee9baeea73186c
Merge "Set default_network in tempest.conf"
Merge branch 'master' of github.com:openstack-dev/devstack