ensure-zookeeper: better match return code

Every time I open up the Zuul console for certain jobs it unrolls the
pre-playbook section in the console because it looks like this task
has a "FAILED" status.

pkill man page says that 0/1 are "process killed" and "no match"
respectively; set failed_when to >1 so these return codes don't cause
spurious failures.

Change-Id: I23112b1101c991c1714d69f7568f83c2dcd605dd

roles/ensure-zookeeper/tasks/main.yaml

@@ -52,7 +52,11 @@
 
 - name: Ensure Zookeeper not running
   command: pkill -f zookeeper
-  ignore_errors: true
+  register: _pkill
+  # 0 is killed
+  # 1 is no process matched
+  # >1 is some sort of actual error
+  failed_when: _pkill.rc > 1
   become: true
 
 - name: Start Zookeeper