In commit 07d7cfd3, people add ${DEBUG} == "true" in file
cluster/saltbase/salt/generate-cert/make-ca-cert.sh
But the default value for DEBUG is not set. In that commit, it set the value
of DEBUG in cluster/ubuntu/util.sh where it call this script. When using this
script in saltstack to bring up cluster in other cloud platforms, it will fail
to generate the cert since we set set -o nounset in make-ca-cert.sh and var DEBUG
does not set. Set a default value for DEBUG here will fix this problem.