Reduce test resource consumption cephfs-nfs

Reduce test concurrency to 1, and limit the tests to only those
tagged with 'backend' to hopefully reduce the resouce consumption
causing timeouts and flakiness in cephfs-nfs standalone jobs.

Change-Id: I777e8137c5a2b8f2a03667faf413ae65b1170d48
diff --git a/zuul.d/manila-tempest-jobs.yaml b/zuul.d/manila-tempest-jobs.yaml
index 02d07fe..6a59fda 100644
--- a/zuul.d/manila-tempest-jobs.yaml
+++ b/zuul.d/manila-tempest-jobs.yaml
@@ -487,9 +487,11 @@
     parent: manila-tempest-plugin-cephfs-nfs-base
     branches: *ubuntu_jammy_test_image_branches
     vars:
+      tempest_concurrency: 1
       # TODO(gouthamr): some tests are disabled due to bugs
       # IPv6 Tests: https://bugs.launchpad.net/manila/+bug/1998489
       tempest_exclude_regex: "(^manila_tempest_tests.tests.scenario.*IPv6.*)"
+      tempest_test_regex: '(^manila_tempest_tests.tests.scenario)(?=.*\[.*\bbackend\b.*\])'
       devstack_localrc:
         CEPHADM_DEPLOY: True
         CEPHADM_DEV_OSD: True