Use set -e for validation-scripts/all-nodes.sh

Use set -e for validation-scripts/all-nodes.sh

Update this script to use 'set -e' for all commands except
the ping checks themselves which we allow to fail so we
can give enhanced output.

This resolves an issue where some commands could fail and cause
an undetectable error (the scripted exits with success) thus causing
a case where Heat won't detect any network errors at all. This
was recently the case with git commit 45be848 where we switched
to use python -c 'import ipaddr' which wasn't even installed as a
library on our overcloud nodes, thus causing all network validations
to silently fail.

Change-Id: I40ed6a537136e866357cc0d9304e905afdd28522
Depends-On: Ia617f44b4673b89202e5e5cdcac9b50f46b3e6c8
Related-bug: #1551048