configure-istio: Improve error message when no ingress ip could be determined

[christian-kreuzberger-dtx]

After installing istio it takes a couple of seconds before an Ingress IP can be determined. This is okay, though our error message suggests that something is wrong.

$ ./configure-istio.sh
Configure Istio and Keptn
INGRESS_IP is empty. Make sure that the Ingress gateway is ready

I believe we should adapt the code-snippet below

examples/istio-configuration/configure-istio.sh

Lines 9 to 13 in 8c8334d

	# Check if IP-Address is not empty or pending
	if [ -z "$INGRESS_IP" ] || [ "$INGRESS_IP" = "Pending" ] ; then
	echo "INGRESS_IP is empty. Make sure that the Ingress gateway is ready"
	exit 1
	fi

such that it prints:

1. The kubectl command to verify what is going on
2. General advice, that if IP adress is pending for more than 5 minutes, something is wrong (e.g., no load balancer is available)
3. Link to the istio docs for more information