Only run zfs command if command is present
Only run zfs command if command is present

zfs package is only installed under certain options, so
only run the zfs command in scripts-library if the command is
installed

Change-Id: I2ecfd4bac657f3bd2d901b85909e73f77e39e510

bunchc/openstack-ansible,openstack/openstack-ansible,bunchc/openstack-ansible,openstack/openstack-ansible,stackforge/os-ansible-deployment,stackforge/os-ansible-deployment