Add internal path for loadtest

[mikkeloscar]

This extends the existing load test to test internal path in the cluster e.g. to test the pod network via the path illustrated below:

The purpose of this is to test the pod network during a cluster upgrade e.g. during flannel rotation which can cause a short drop of network availability (#9272).

To make the test work it further improves the load test

• Fix checking if daemonset pods are up to date in the wait-for-update.py script
• Prevent scale-down of the vegeta/load test client to ensure it always running also when flannel is rotated.
• Use kubectl apply instead of create to make it simpler to run start-load-test.sh locally.

TODO

• Drop flannel change which is currently there to validate the test would catch it.

[MustafaSaber]

👍

[mikkeloscar]

👍

[demonCoder95]

👍