Redirect the log of the slave to /root/jenkins-slave.log

So a failure on that script can be investigated.

Change-Id: If1834f8fda02881c88e9ce5e422b7acd963fcf50
diff --git a/gce/jenkins-slave.sh b/gce/jenkins-slave.sh
index ddb5a78..27a5c2d 100644
--- a/gce/jenkins-slave.sh
+++ b/gce/jenkins-slave.sh
@@ -20,6 +20,7 @@
 # We ignore errors on this script as we want to retry on errors.
 
 cat > /root/jenkins-startup <<'EOF'
+&> /root/jenkins-startup.log
 
 # Wait until the machine is set-up on creation
 while ! (id ci >&/dev/null) || [ ! -f /home/ci/node_name ]; do