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