solum f20 gate is failing due to upstream dep.
solum f20 gate is failing due to upstream dep.

switch gating devstack tests to trusty instead
until upstream problem is resolved.

Change-Id: I2801b8fd5104e403226b30fb23a022ebd0fa6342

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