Move oslo default test node to trusty.
Move oslo default test node to trusty.

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

Change-Id: I65b60c4a2df69b5e41162748b95e8ea47be4cd14

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