libvirt: Fix nova-libvirt-cleanup command

This change addresses an issue in the nova-libvirt-cleanup command,
added in I46854ed7eaf1d5b5e3ccd8531c963427848bdc99.

Check for rc=1 pgrep command, since a lack of matches is a pass.

Also, use bash for set -o pipefail.

Change-Id: Iffda0dfffce8768324ffec55e629134c70e2e996

ansible/roles/nova-cell/tasks/libvirt-cleanup.yml

@@ -14,12 +14,14 @@
   register: container_facts
 
 - block:
+    # NOTE(mgoddard): pgrep exit status 1 means no match.
     - name: Check if there are any running VMs
       become: true
       shell:
         cmd: >
           set -o pipefail &&
-          pgrep -l qemu | awk '!/qemu-ga/  && !/qemu-img/ {print $1}'
+          { pgrep -l qemu || test $? = 1; } | awk '!/qemu-ga/  && !/qemu-img/ {print $1}'
+        executable: /bin/bash
       register: running_vms
 
     - name: Fail if there are any running VMs