infra-prod: fix infra-prod-service-zookeeper soft dependency

This is a typo from the job shuffle in
I8f6150ec2f696933c93560c11fed0fd16b11bf65 -- this should be a soft
dependency.

It is currently causing periodic jobs to fail

Change-Id: Ia420e74a1d64b12b63b1697e61992c46119451dc

zuul.d/project.yaml

@@ -543,6 +543,7 @@
               - name: infra-prod-letsencrypt
                 soft: true
               - name: system-config-promote-image-zookeeper-statsd
+                soft: true
         - infra-prod-service-zuul: &infra-prod-service-zuul
             dependencies:
               - name: infra-prod-service-borg-backup