zuul: explicitly enable c-bak in the lvm-lio job

Due to memory contraints, until c-bak memory usage is investigated
more in depth, c-bak may be disabled by default in the parent job.
So explicitly enable it in this job.

Change-Id: I89ad6fdd2e7f861e1830118376102e06fcb7c3fc

.zuul.yaml

@@ -128,6 +128,8 @@
               volume_revert: True
       devstack_services:
         barbican: true
+        # explicitly enable c-bak, as it may be disabled in the parent job
+        c-bak: true
       tempest_plugins:
         - cinder-tempest-plugin
     irrelevant-files: