CI: Add "may_fail" step to execute flaky steps

Add a generic may_fail CI XML step option that indicates the CI step may fail, but the CI would still be marked as succeeded (instead of skipping all steps and mark as failed). The cause of this failure depends on the specific step, e.g., Iperf failing because of low throughput. This allows to clean up some code and e.g. remove the command_fail option of Custom_Command, which is then superfluous (and less generic). An inconvenience is that those may_fail steps will be marked as "failed" in the HTML. Later cleanup can correct this.

The may_fail is used for executing FHI7.2 testing to push 700 Mbps in DL (close to the maximum).

Fix some bugs and add some clean up, see commit messages.