Fail fast during advanced networking test

This gives us the opportunity to discern whether the network
was sane prior to the server operation under scrutiny.

Change-Id: Iacc4f0307019efafda0de3abb0fa71ad13770850
Closes-bug: #1312841
Merge "Fail fast during advanced networking test"
Merge branch 'master' of https://github.com/openstack/tempest