Update scripts-library.sh to grab output when command exists
Update scripts-library.sh to grab output when command exists

This patch updates scripts-library.sh to only grab output from btrfs,
nspawn, and iptables when the commands exist in order to aid in
the troubleshooting of (real) failures.

Change-Id: I5c8677d3e8d1e1c6a6486654d532a899b5d0cf29

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