Tests: reduce concurrency for cephfs-nfs job

Reduce it to 2, which should ensure that this job does
not fail for lack of resources in devstack gate.  We can
bump it back up later if we have test infrastructure improvements,
but in the mean time there is no point holding up fixes and
enhancements to Manila itself because of vulnerabilities in
the test framework itself.

Change-Id: Icf18d737859171510b8f7db6e088d63041fd0bf4
diff --git a/zuul.d/manila-tempest-jobs.yaml b/zuul.d/manila-tempest-jobs.yaml
index 0727b43..e115b41 100644
--- a/zuul.d/manila-tempest-jobs.yaml
+++ b/zuul.d/manila-tempest-jobs.yaml
@@ -339,6 +339,7 @@
       - openstack/devstack-plugin-ceph
       - openstack/neutron-dynamic-routing
     vars:
+      tempest_concurrency: 2
       devstack_plugins:
         devstack-plugin-ceph: https://opendev.org/openstack/devstack-plugin-ceph
         neutron-dynamic-routing: https://opendev.org/openstack/neutron-dynamic-routing