Add a nicer trace prompt for $PS4

Since we are standardizing on bash for shell scripts, we may as well
tell it to use a trace prompt that is vastly more useful for debugging.

This patchset changes $PS4 to prepend the script name and line number to
every line output in a debug trace.

Change-Id: I4ec5c0253295724a9580f86fd30082acd89d155d

scripts/devtest.sh

@@ -3,6 +3,13 @@
 # Demo script for Tripleo - the dev/test story.
 # This can be run for CI purposes, by passing --trash-my-machine to it.
 # Without that parameter, the script is a no-op.
+
+# Set PS4 as early as possible if it is still at the default, so that
+# we have a useful trace output for everything when running devtest.sh
+# with bash -x ./devtest.sh
+if [ "$PS4" = "+ " ]; then
+    export PS4='${BASH_SOURCE}@${LINENO}: '
+fi
 set -eu
 set -o pipefail
 SCRIPT_NAME=$(basename $0)