Fix error logging of dev/functions

In case of failures in kayobe-overcloud-centos, the error message fails
with:
kayobe/dev/functions: line 569: LOGDIR: unbound variable

Example:
https://zuul.opendev.org/t/openstack/build/ce1fadc3ee6d4842a599da57a670cc18

This can be reproduced with:
set -eu

if [[ -n ${LOGDIR} ]]; then
    echo "LOGDIR set"
else
    echo "else"
fi

Fix the error reporting with assigning an empty string to LOGDIR by
default.

Change-Id: Ieef73950f89e4dfb727ddc59ef2750d9b81f3c58