Fix experimental job in ironic-staging-drivers
Fix experimental job in ironic-staging-drivers

the 'ironic-tempest-plugin' builder actually requires 'concurency' var
that was missing from the original commit.

Change-Id: I34c87b89c1d6f7b07cd79c595e2f904b5a77a33d

openstack-infra/project-config,openstack-infra/project-config