Move nova default test node to trusty.
Move nova default test node to trusty.

This change will run all nova and nova specs python27 unittests, doc
builds, and flake8 jobs on trusty nodes.

Change-Id: Ib7ff5694c40e5439dbd9fb60cbfb39cca17cd904

dongwenjuan/project-config,openstack-infra/project-config,Tesora/tesora-project-config,noorul/os-project-config,dongwenjuan/project-config,coolsvap/project-config,citrix-openstack/project-config,Tesora/tesora-project-config,osrg/project-config,open-switch/infra_project-config,coolsvap/project-config,citrix-openstack/project-config,open-switch/infra_project-config,anbangr/osci-project-config,openstack-infra/project-config,osrg/project-config,noorul/os-project-config,anbangr/osci-project-config