Perform merge-check jobs at low precedence
Perform merge-check jobs at low precedence

Now that Zuul supports precedence passthrough to the merge-check
pipeline, merge-check pseudo-jobs should yield to more important
pipelines when necessary.

Change-Id: I8a2c1621a8326d1a5c2049065058a2bd699b9194

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