Create LOGDIR if it doesn't exist

Create LOGDIR when LOGFILE is not set.  This fix is rather blunt
as logging setup will be further tweaked as
https://github.com/openstack/qa-specs/blob/master/specs/devstack/devstack-logging-and-service-names.rst
progresses.

Change-Id: I4a574f295eb6d55c7196ec563bd356d6c0bc8833

stack.sh

@@ -352,6 +352,10 @@ TIMESTAMP_FORMAT=${TIMESTAMP_FORMAT:-"%F-%H%M%S"}
 LOGDAYS=${LOGDAYS:-7}
 CURRENT_LOG_TIME=$(date "+$TIMESTAMP_FORMAT")
 
+if [[ -n ${LOGDIR:-} ]]; then
+    mkdir -p $LOGDIR
+fi
+
 if [[ -n "$LOGFILE" ]]; then
     # Clean up old log files.  Append '.*' to the user-specified
     # ``LOGFILE`` to match the date in the search template.